There are some Arduino/ATMega-based Synths, as well as MIDI and CV (control voltage) "shields" to allow for more experimentation.
"Audino"- an code and other Arduino projects at http://code.google.com/p/tinkerit/http://www.arduino.cc/playground/Main/SID-emulator
(if you like the SID chip)http://www.avishowtech.com/mbhp/mbhp_core32r1.html
(if you like the ARM chip)
SN Voice is another interesting project, although some of the components besides the SN76477 are hard to find. There are forums on multiple sites, search Google for SN Voice