ArduTouch
Build it, and it works!
Way low cost (currently $30.)
It comes with a pre-programmed music synthesizer that makes way cool sounds and music and noise.
An ArduTouch library is available for programming in more super nice synthesizer features, and for making your own synthesizers. The library comes with examples (with more to come) that serve as a nice tutorial on how to create your own ArduTouch synthesizers (which are Arduino sketches).
ArduTouch is really just an Arduino Uno clone with a small amplifier & speaker, with a PCB touch keyboard, a couple of extra buttons, and a couple of pots (for playing music and controlling the sounds). The ArduTouch Arduino Library is where the real magic comes from!
For those who want to learn more, the documentation teaches the basics of Digital Signal Processing for audio generation (more documentation coming soon).