Small PCBs (is there a better term than "small PCB" for the sorts of PCBs that are specifically designed to be hacked and modified -- they have pins that are perfectly aligned to be plugged into a solderless breadboard, or they have a prototyping area on-board?)
Designed to be used as a components.
Rather than buying a raw microcontroller, many people buy a small PCB that includes the microcontroller and other essential Popular Parts on a tiny little microcontroller development board. Many such boards are demo board, easy to plug into your solderless breadboard (or your custom PCB) that has just a few more things that make your project special.
Later I learn: "A plugin is a small electronic plug-in module capable of being plugged directly into a breadboard, that fulfills a (usually simple) function." I also discover some people call these things "breadboardable" or "DIP-like" or "breadboard compatible".
Occasionally called a "breakout board" or "adapter board".
FIXME: should we split up this page into one page "processor development board" for boards with a processor on it, and another page for boards with sensors or etc. but no processor?
- DsPIC30F 5011 Development Board
- USB Bit Whacker: a USB Enabled PIC Development Board
- ADuC832 Development Board
- Connectors mentions "common pitfalls of Micro Development Boards"
Form factor comparisons and reviews
What are standard interfaces between a "CPU board" and a "I/O board"? If none of the standard interfaces are quite what I want, what are good things to keep in mind when designing a new interface?
- "Rethinking the Arduino hardware interface" describes the differences between the "JeeNode form factor and I/O connectors" and the "Arduino Shield form factor and I/O connectors" (both systems are fully compatible with the standard Arduino IDE software).
- Mark has a brief rant about form factors
- "Acme Daisy Cabling System"
Demo board comparisons and reviews
- Boris Kourtoukov. "Body boards: A guide to wearable microcontrollers". Make Magazine 2015-March.
- Alasdair Allan. "A Smorgas-Board: Wearables and IoT are driving diversity in the microcontroller market". Make Magazine 2015-March.
- A decision on Basic Stamps and Parallax -- Basic Stamp vs. Arduino vs. Propeller
Lists of demo boards
- Which Board is Right for Me?
- "Microchip PIC Alternatives" lists many demo boards.
- Instructables: Modules, bootloaders, and "hidden" microcontrollers also lists many demo boards.
- Some people are looking for a "a plugin PCB, with a DIP-like interface" that can run Linux: "Developing Embedded Systems (uC-Linux) on Breadboards." Does such a thing exist ?
- "10 Digital Tools Democratizing Making"
Demo boards, in no particular order
boards designed to be wearable
What features are nice to have on wearable electronics? (both microcontroller boards and other electronics)
- fabric-friendly connections -- snaps, metal eyelets, or ... ?
- low-power, so we don't need to carry a heavy battery
- To be comfortable sewn into fabric worn on a body with a shape that is not merely not flat, but actively changes shape, it needs to either be flexible, or be relatively small boards with no sharp corners.
- parts that are not washable -- batteries -- are easily removeable.
- ... what else?
(FIXME: add dimensions to these boards)
LilyPad Arduino SimpleSnap 
TinyLily Mini 
Hyejung Kim; Yongsang Kim; Binhee Kim; and Hoi-Jun Yoo. "A Wearable Fabric Computer by Planar-Fashionable Circuit Board Technique".
PIC based boards
- Ready to use daughter boards based on PIC microcontrollers. Each model is designed to meet different price/performance requirements with the smallest possible footprint
- OOPic: The Object-Oriented PIC. The "ooPIC-S" board is a PCB with a large prototyping area and a PIC16F877. The "ooPIC-C" is a tiny stamp-like PCB that plugs into a solderless breadboard. Both hold a PIC16F877, a Flash memory chip, 3 LEDs, a voltage regulator, and a RS-232 tranciever). Both run the "ooPIC III+" firmware.
- BeagleBoard PIC module plugs into standard solderless breadboard
by Brian Schmalz ... "The UBW board ... contains a Microchip PIC USB-capable microcontroller, headers to bring out all of the PICs signal lines (to a breadboard for example) ... costs about $15-$20 to build and is powered from the USB connection. ... All tools used to design this project are free, and the design is open to anyone to use for whatever they wish. Please build your own and improve upon it! If you don't want to build your own, you can buy them assembled and tested ... SparkFun hosts a USB forum where the UBW is a common discussion topic."
- http://microcontrollershop.com/ has a bunch of development boards (click on the type of CPU in the left column ... then when the sub-menu opens underneath the CPU type, click on the "boards" in that sub-menu ...) such as, for example, the " PIC18F67J60 Web Server Prototyping Board, Ethernet, RS232
- Dontronics sells a variety of PIC-oriented proto boards and development boards (as well as development boards for several other popular microprocessors).
- iRX Prototyping Board: a simple but malleable prototyping board for PIC based projects. It features the EEPROM programmable PIC16F84A, an MAX233 for RS232 port and infrared I/O. About half of the board is an empty prototyping area. Compare: Atmex.
- Sirius microSystems has several "easily hackable" development boards, mostly based on Microchip PICmicro. Also posts circuits and schematic diagrams under a "Creative Commons Attribution-Noncommercial-Share Alike 3.0 License", making them "open hardware".
- (FIXME: there may be a few more mentioned at http://massmind.org/techref/microchip/piclinks.htm that ought to be listed here.)
- microEngineering Labs, Inc. specializes in development tools for Microchip PIC® microcontrollers." Lots of completely empty boards designed to be stuffed with various PICmicro chips and associated components ... such as the $19.95 PICPROTO-USB -- compatible with 28 and 40-pin devices, this board is equipped with additional USB circuitry and connector for use with the USB-equipped PICmicros like the 16C745 and 18F4550.
- E-Blocks plug and play modules for PICmicro prototyping. Compatible with many PIC16F and PIC18F MCUs.
- E8100 based on dsPIC33F OEM single board sized in credit card footprint
- E8140 based on PIC18F4680 single low cost board sized for minimum footprint
- E8160 based on PIC18F2510single low cost board sized for minimum footprint
- PIC BASIC Stamp
- PIC C Stamp intended to be a step up from the BASIC Stamp; uses a PIC18F6520 (32 KByte flash, 2 KByte RAM)
- Protean Logic TICkit -- The module is directly programmable in a library-centric Functional Basic
- the HOODMICRO
- SerReg (FIXME: is this really a development board?)
- LiniStepper designed to drive stepper motors
- Communitary Universal Microcontroller Programmer
- School electronics has many "system module designs"
- AXEme Picaxe Construction Kit http://www.qrpme.com/ includes a small solderless protoboard
- There's a huge number of AVR-based Arduino-like boards at Arduino Links, most of which are breadboard-friendly.
- Light Blue Bean - Arduino-compatible board; "upload the sketch over Bluetooth LE"
- WeIO "WeIO is an Open Source Hardware IoT Board Programmable from a Web Browser"; "Tiny open-source module aims to make IoT apps easy"; "WeIO"; "WeIO in Details". The IDE is hosted entirely on the WeIO Board; you can do software development entirely in a web browser on any laptop.
- ARM links - there are a growing number of Arduino-like embedded ARM boards, offering more CPU power than Arduino but many of the same capabilities.
There seem to be 3 main categories of ARM boards: (is there a better way to categorize them?)
- ARM boards intended to run a very small program (possibly using a simple real-time operating system RTOS) written in a low-level language. These tend to be smaller and run longer on batteries, so are better for wearables.
- ARM boards intended to run Linux (which requires a MMU) and display graphics on a computer monitor (which requires much more hardware and drains batteries faster than the above boards).
Most (all?) of the Motherboards that run Linux are based on a high-end ARM processor.
Other demo boards
- AMPS is a modular prototyping system for microcontrollers from Technological Arts, focused primarily on HC11, HC12, S12 and S12X chips from Freescale. It's like lego for microcontrollers because all the connectors and form-factors of the "Adapt" cards are standardized and interchangeable, enabling you to arrange them in various ways. (http://www.technologicalarts.ca/catalog/index.php?cPath=50)
- Esduino (http://www.technologicalarts.ca/catalog/index.php?cPath=50_166) is an Arduino hardware-compatible board utilizing a Freescale 9S12C 16-bit microcontroller
- $4.30 TI MSP430 LaunchPad
- Bread Board Power Supply -- a tiny little board that fits between a wall-wart and plugs into a solderless breadboard to supply regulated power. It accepts a wide variety of wall-warts because its full-wave rectifier and LM317 handle a wide range of AC and DC power supply, and its screw terminal headers let you plug in the wires.
- A huge list of Makezine kits, a few of which ("Daisy MP3 player", some of the RepRap projects, "The IR-ritator", etc.) use PICmicros, a few others that use Atmel AVRs (TV-B-Gone, etc.), one that uses a Freescale processor ...
- AVR BlinkM, which can be used as a AVR ATtiny45 development board
- SpinStudio, a system designed for development using Parallax's Propeller(TM) Microcontroller.
- AVR Atmex: An entry-level Atmel Experimentation System. less than $10; includes Atmel ATtiny2313; a DB-9 and MAX232 for serial communication and downloading new programs (using a bootloader); LED. About half of the board is an empty prototyping area. Meant to be better than the iRX board.
- "NanoCore12", much like the BASIC Stamp but using a Motorola/Freescale HCS12 instruction set http://www.nanocore12.com/ from Technological Arts. It has a free object-based programming environment for Windows, called nqBASIC (http://www.nqBASIC.com)
- Micromint http://www.micromint.com/ sells little boards with DIP pinouts (and so can plug into protoboards) based on ARM7, Atmel AVR, and 8051 cores ...
- "The uCsimm module is a microcontroller module built specifically for the uClinux Operating System. It stands an inch high, with a standard 30-pin SIMM form factor. ... The uCsimm is driven by a Motorola DragonBall 68EZ328 processor, and comes well equiped with 2 MB of FLASH and 8Mb of DRAM. We have also included a 10Base-T ethernet and RS 232 high-speed serial." http://uclinux.org/ucsimm/
- "The Minotaur BF537 is a sub miniature Computer On Module ... 26.5mm x 26.5mm x 4.2mm ... 10/100 Ethernet MAC and PHY onboard ... uClinux support" http://www.camsig.co.uk/products.htm
- "the Hammer": a Samsung S3C2410A ARM9 CPU board in a 40-pin DIP package, ready to plug into a solderless breadboard. Pre-loaded with Linux. ... discussion forum ... Hammer wiki (FIXME: add to WikiNode) ...
- uC Hobby occasionally discusses various development boards http://www.uchobby.com/
- Some people use the AT90USBKey as a prototyping board.
- "Picotux was the smallest computer running Linux in the world. ... 35 mm × 19 mm × 19 mm and just barely larger than [its] RJ45 connector" and its DB9 serial connector. "... Two communication interfaces are provided, 10/100 Mbit/s half/full duplex Ethernet and a serial port with up to 230.400 bit/s. Five additional lines can be used for either general input/output or serial handshaking. ... 55 MHz 32-bit ARM7 Netsilicon NS7520 processor, with 2 MB of Flash Memory (750 KB of which contains the OS) and 8 MB SDRAM Memory. The operating system is µClinux" -- Wikipedia: Picotux
- AVR StAVeR-24M32 "a microcontroller module based on an Atmel ATmega32 in a small DIP24 package with a standard pin out."
- Copper (AVR32 Stick) "Copper is an Atmel AVR32 UC3B1256 mounted on a DIP32 breadboard friendly module, giving you the smallest AVR32 development board available (and one of the cheapest!). Features integrated voltage regulator, USB connector, and in-system programming via the USB port. ... The board design and schematics are available under the Creative Commons Attribution Share-Alike 3.0 license. ... 256KB of flash, 32KB of RAM, on board USB, up to 60MHz CPU clock ..."
- Digi XBee radio transcievers
- Particle.io Photon (formerly Spark.io Photon) - includes ARM Cortex M3 and Wi-Fi
- Particle.io Electron (formerly Spark.io Photon) - includes ARM Cortex M3 and 2G/3G cellular
- The WunderBar  - includes Wi-Fi
- ESP8266 Wi-Fi serial transciever
- littleBits has a huge number of small demo boards that connect and do things without programming, without a microprocessor.
- Some technical details for people who want to build their own customized littleBits modules (and perhaps after that get your module featured in the the bitLab):
- HDK Tips & Tricks;
- the book by Ayah Bdeir, Matt Richardson: "Make: Getting Started with littleBits: Prototyping and Inventing with Modular Electronics";
- "The littleBits Eagle Files Repository"
- "more littleBits github repositories"
- the "Hardware" section of the littleBits discussion forum
- some measurements of exactly how much power various littleBits require (useful for estimating battery runtime): [http://discuss.littlebits.cc/t/specs-for-the-battery-cable/152
Specs for the battery cable"]github: manitou48: littleBits power measurements
- SAM labs
- ["SAM: The Ultimate Internet Connected Electronics Kit"]
- "SAM Is A Wireless Electronics Kit That Wants To Make Coding Connected Objects Super Simple" says "The SAM hardware kit schematics and board designs will definitely be open sourced."
- Samlabs wireless electronics kit
Demo boards are one of several techniques for rapidly prototyping circuits -- see Techniques#Circuit construction .28Prototyping - Other than custom PCB.29 for a few more.