I recently got the iTrip FM transmitter, and let me tell you, it BLOWS EVERY OTHER TRANSMITTER OUT OF THE WATER! It is the most impressive transmitter I've EVER seen! I cannot say enough good things about this! I mean, it still is FM-transmission, but if you don't have a line-in on your deck (or a cassette adapter) then this is for you!
I even can play it at home (while on the couch w/ the sager) and transmit to my stereo system many feet away w/o fuzz or static.
I'm not sure it works with other mp3 players, as it gets it's power from the audio/remote microjack so it doesn't use batteries... it also uses mp3's to control it (change to any frequency).
A very good design overall.