sdiy.info — The SDIY wiki is for learning and sharing knowledge about making, modifying, or repairing electronic musical instruments and related equipment.

buildmusicthings.com — Building and hacking open source synths and synth kits.

celticpeasant.com — Synth DIY music and its relationship to life, the universe and everything.

electronicpeasant.com — Diy Music Electronics

fpga.synth.net — FPGA synthesizer resource site. Based mostly around a wiki, but with lots of useful code and examples. included complete FPGA synthesizers.