Synth designer Ray Wilson is now selling kits for his “Weird Sound Generator Reborn” standalone instrument module. It’s a quick build and makes for some very cool noises. There’s even professional-grade front panels available for use on the enclosure.
Ray’s site is a great resource for learning about synthesizers and electronics in general. His circuits are well documented with a welcome sense of humor.

Weird Sound Generator kits – Link

Music from Outer Space – Link

DIY Synth pages
Synthesizer DIY pages of Ren̩ Schmitz РLink

Cigar Box Synth
Synthesizers @ MAKE – Link

Super-simple DIY synth plans – Link