This video, via DINSync, demonstrates modifications to the Gakken SX-150 synthesizer – a cheap analog synth that can be easily hacked:

Here’s what’s got to be one of the best bargain analog synthesizers out there. It’s the Gakken SX-150, a stylophone styled analog that can be found on ebay for $40. The machine has to be assembled, but there’s no soldering involved. It’s just a matter of putting the PCB in the case and connecting the power line, pretty simple stuff really. It’s very much a gimmick but since it really is a true analog it’s fairly easy to modify it into something useful.

Details on the hacks at the DINSync site.