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.

cgs.synth.net —  A resource for modular synthesizer builders that also offers PCBs for the designs presented.

techman.synth.net — Andy's Modular synth page

chameleon.synth.net — The Chameleon is a truly multi-functional rack unit. You can completely "re-skin" it to be anything you need. It could be a monster filterbank, mastering tool and vocoder, a high-quality reverb, a virtual analogue synth module, an amp-modeller for guitar recordings, a solid multi-FX for mixdown, a dance music machine for a DJ gig, a gungy monosynth, warping delay-FX module... it's whatever you need, and it just takes a minute.

diy.synth.net — A hub for Synth DIY event details around the world and a gallery for pictures from each!

arptech.synth.net — This site is a source of technical information on vintage analogue music synthesisers once designed and manufactured by ARP Instruments, Inc.