Pianocade, an open source arcade style chiptune synth and midi controller.
Google’s MoogDoodle. Truly awesome. Happy birthday Bob!
NTH hackable synth.
Commodore 64 keyboard overlay and synth cartridge. C64s had pretty amazing synths inside.
Muson Synthesizer, 1979.
Quintronics Drum Buddy, a light activated analog synthesizer / drum machine.
Hohner Multimonica II - early German synth.