iPod is the only player compatible with any songs bought from the iTunes store.
If you just want a mac compatible player then iAudio blows the iPod away for audio quality. It just acts as a hard disk and you drag and drop music on.
i have an ipod photo 40g. i bought it for 499 when it came out... now it is acting funny, so i am looking for a new mp3 player... I have an iBook G4 running Mac os 10.4.5 and i was wondering if there are any other mp3 players that work with itunes... i think i'll probably get an ipod but i want to keep an open mind....