BMPX is an experimental multimedia player which is extremely easy to use, including many features such as streaming podcasts, last.fm support, online radio stations, audio cd/artist info and lyrics retrieval, and much more.
Exaile is a media player aiming to be similar to KDE's Amarok, but for GTK+. It incorporates many of the cool things from Amarok (and other media players) like automatic fetching of album art, handling of large libraries, lyrics fetching, artist/album information via Wikipedia, last.fm support, optional iPod support.
Audacious is a fast and lightweight music player which is an evolutionary clone of the Beep Media Player, XMMS, and the classic WinAmp. Supports most all audio formats, play lists, last.fm plug-ins, and any theme designed for it's predecessors.
Banshee is an audio management and playback application for the GNOME Desktop, allowing users to import audio from CDs, search their library, create playlists of selections of their library, sync music to/from iPods, and burn selections to a CD.
Amarok is a music player and organizer for the KDE Desktop that has many amazing features such as grabbing aritist/album info from wikipedia, playlist creation and browsing, and iPod/iRiver support.
Rhythmbox is a very easy to use music playing and management program which supports a wide range of audio formats (including mp3 and ogg). Originally inspired by Apple's iTunes, the current version also supports Internet Radio, iPod integration and generic portable audio player support, Audio CD burning, Audio CD playback, music sharing, and Podcasts.
Noatun is an audio and video player for KDE and the arts sound system. It can play a range of different formats and can support various plugins such as alarm clocks, lyric readers, and interchangable interfaces.