I have a 512MB Napster player that I got "free" with a subscription a while ago.
Drivers needed? None. It supports two modes of file transfers. You can have it work as a USB flash drive where you just copy files to it and it indexes them and plays once disconnected from the computer. It also supports MTP mode, for copying on a file level, and confirming the files copied without issue.
Mount the drive and low level erase it in Linux (dd if=/dev/zero of=/dev/sda), as soon as you disconnect the player, it automatically realizes it needs to format its flash memory, does so, and is ready to go.
The OEM is DNC Tech (www.dncmultimedia.com). Unfortunately, they are out of the MP3 player business.
Why can't more MP3 players be like this, where it doesn't require special drivers or software (Zune player, iTunes) to work? I'd like to see a player with a non trivial size (30-60 gigs) be like this, and the last player I saw that was like this was an Archos player. The Archos player had quality issues though (a metal burr on the case, randomly would power off, corrupting its FAT filesystem), so had to return it.