A software-defined radio (SDR) is a radio communication system where components historically implemented in hardware (for example, mixers, filters, amplifiers, modulators/demodulators, detectors, etc.) are instead implemented by means of software on a personal computer or embedded system.
Several people working on software-defined radios have released open-source designs (open hardware).
One application of software-defined radio is radio astronomy.
Some people build RFID Readers using software-defined radio so a single reader can read many kinds of RFID tags, perhaps even some not yet invented.
- GNURadio wiki
- Wikipedia: GNU Radio
- Wikipedia: software-defined radio
- HackRF, an open source SDR platform by Michael Ossmann  
- Signal Identification Guide wiki
- Some of the most cutting-edge work in software defined radio is being done by radio amateurs   
- High Performance Software Defined Radio: An Open Source Design  
- Jonathan Naylor, G4KLX. "A Standard for an SDR to External Programs Interface". Some discussion at Cross Country Wireless and the Cross Country Wireless group.
- The ARRL has a lot of information about software-defined radio:
- Andy Talbot G4JNT. SDR Projects.
- Dave Robinson WW2R. SDR Projects.
- Hackaday occasionally mentions software-defined radio:
- Alas, the open-source µWave SDR Project seems to have gone offline (µWave SDR Project archive).