Electronics for Music is a haven for synth-diy enthusiasts. EFM sells low-cost printed circuit boards for analog synthesizer projects. They currently have a fairly limited line-up that includes the WildCat modular midi synthesizer PC board, and the TomCat midi synthesizer PC board.
EFM Synthesizers is located in Houston, Texas USA. EFM has been on the web since 1996 selling boards and kits to the DIY synthesizer community. The site includes schematics for ARP, Moog, Oberheim, Korg, and EMS-type voltage controlled filters, oscillators, amplifiers, and everything else you might need to build a modular synth.
EFM projects are for experienced DIY builders, because project often require mods to the PCBs, and documentation assumes a high level of electronics experience.
The site also features several free VSTi synths and effects, including a version of the ARP Odyssey, ARP Axxe, Minimoog, and Prophet 600.