The Melloman - DIY Mellotron, via del.icio.us and probably a buncha other places.
TJ wanted to make one of these for use in Aloha, but he ended up getting the job done with a sampler. But imagine using real honest-to-god cassette tapes for loops and it has a creepy casio-meets-bad-acid-trip sound going on. Maybe you could even work in some kind of echoplex-style tape delay into the package for something truly mysterious.
What would happen if you did some serious circuitbending on that thing???