This is used in [The Programmable Chip EEG project at sourceforge.net].
The digital Potentiometers made by analog devices (AD5204) has 255 positions can be adjusted by the microcomputer that can receive commands from the computer. This is a great way for analog circuitry to have digital control. This is a chip with 4 digitally controlled POTs and the pots can be daisy chained to have multiple chips controled by a single SPI interface.
AD5204—4-Channel with daisy chain ability AD5206—6-Channel without daisy chain ability Potentiometer Replacement 10 kV, 50 kV, 100 kV 3-Wire SPI-Compatible Serial Data Input
Digikey Part Number: AD5204BN10-ND
AD5204 - 4-Channel, 256-Position Digital Potentiometer
- Free samples: http://www.analog.com/en/prod/0,,761_797_AD5204,00.html#price
- Digikey: http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=1480&Row=284443&Site=US (10k version - $5.02)