There are a good number of mp3 players available out there on the market. Personally my favorite has to be the iAudio X5 (which I like over mostly all of the other mp3 players) Karma came in at a close second with its extremely good sound quality, but it was just a little too ugly and plasticky.
iAudio X5(L) (One of the best sounding headphone outs, jet black with many nice features like video, pictures, line in/mic recording, fm radio, etc...)
Rio Karma (Discontinued and ugly, but it is still to this day one of the best, if not the best sounding mp3 players out there)
Apple iPod (Easy to use, simple integration with iTunes, the spin click wheel thing, has a pretty decent line out for headphone amplifiers)
These are mp3 players I have never tried but are pretty popular themselves.
iRiver H10, iRiver H320, iRiver H340, Sony HD-3, Sony HD-5, Zen Micro, Zen Sleek,