MintySynth is an Arduino-compatible synthesizer/sequencer/audio experiment kit that fits neatly in to an Altoids tin. It is intended as an educational tool as well as a fun toy, and is a great way to learn about electronics, programming, and music. Best of all, its open source and hackable!
All components are supplied except the Altoids tin. Some soldering is required, but assembly is straightforward and generally takes one to two hours.
You can either assemble the kit and use it with the preloaded synth software, or you can program it yourself as you would an Arduino or similar microcontroller. The kit includes an FTDI interface inside.
Note: Although the kit doesn't come with the tin, MintySynth is self-contained and works fine by itself if you don't have a tin or don't want an enclosure.