Mobiblu DAH-220 MP3 player: This is a pretty neat, if somewhat useless, hack: it's a 256MB MP3 player in a cassette-tape form factor. But it has a couple of cute features I didn't expect:

"The spindles are connected to a mouse-style encoder that can tell which way they're turning, and how fast. This lets you skip tracks, while it's in a tape deck, by doing a brief fast-forward or rewind."

"The cassette adapter heads work both ways, so you can also record like a regular tape. Put 'em in record mode, then just pop 'em in your tape deck."