According to Wikipedia, at one time mankind used reels of thin flexible tape contained in plastic casings to store and share music. And that's where the creators of the adorably tiny Mixtape Alpha 8-bit synth drew their design inspiration.
Although it was created to be open source and easily hacked, the Mixtape Alpha's not one of those situations where you're just given the schematics and expected to build your own. For just $35 you can actually order one from the Open Music Labs online store. Once they get them back in stock that is.
The synth includes a series of buttons and even a small stylophone keyboard, as well as four voices, four effects, and the ability to play up to five sounds at once. You can even record and playback your creations, or share them with friends like an old-school—wait for it—mixtape. [Open Music Labs via Make]