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.

simmons.synth.net —   The Simmons range of drum synths was unique in sound, design and use, it formed a huge part of the sound of songs in the 1980's and became instantly recognisable.   We hope to try and include user guides and service guides for every peice of Simmons equipment made, but there's not much around, so if you have any thing you'd like to add, please feel free to contact us.  

ppg.synth.net —   These pages are dedicated to the PPG range of synthesizers. 
A range of unique synths that shaped the sound of the 80's and gave the world a new type of synthesiser.