Xine - "xine is a free multimedia player. It plays back CDs, DVDs, and VCDs. It also decodes multimedia files like AVI, MOV, WMV, and MP3 from local disk drives, and displays multimedia streamed over the Internet. It interprets many of the most common multimedia formats available - and some of the most uncommon formats, too."
Free
CDex - Multi format music converter, excellent for making mp3 backup's of your CD collection.
Open Source
Quintessential Player - Great free media player with some excellent skins and plug-ins, a worthy alternative for your media needs.
Freeware
WinAmp - An mp3 player that goes that little further, great free program.
Free version available