I remember seeing a prototype (or something similar) of this kind of device maybe a couple of years ago and thought it was a pretty good idea. For those of you who have old cassette decks (or new ones, even) in your car but have gone all digital as far as your music is concerned, this might be a doodad worth looking into.
It’s basically a cassette tape that holds an SD card loaded up with MP3 files. You can use it on its own as an MP3 player or pop it into a cassette deck and play the files as though they were on the tape itself.
ThinkGeek has it for $39.99, while Chinavasion has it for $22.45. I’m not quite sure how you’d skip about tracks while it’s in your cassette deck but Chinavasion says “Use like a normal cassette in a car cassette player or cassette player,” so I’m guessing that might mean fast forwarding and rewinding just like the good old days.