Linux compatible portable mp3 player

I am looking for recommendations for a portable mp3 player that will work with Linux. Basicly I hope it appears as a USB drive so one can simply copy the music to and from the player. Note:Its for a friend, I have no knowledge and in fact little interest in such devices. I am sure though this has been discussed before on this list. A search on the net produced results that were some years out of date. Lindsay