Most of the codecs available for Linux are not available as separate "codecs", as such. They are usually built into the libraries supporting the core multimedia programs, such as xine, gstreamer, vlc, and mplayer/ffmpeg/libavformat. There are several "cross-pollination" plugins that can let one player use the other player's codec libraries, however. In addition there are many separate format libraries, such as liblame for mp3 and faac/faad for the aac audio format, that can be used both independently and by media players.
Finally, there are a few proprietary codecs that just aren't available as "Free" packages, but can be used in a Linux system. There's a realplayer available for Linux, and some players can use certain win32 codecs, packages of which are available from the mplayer project (note that most of these will not work on 64bit systems).
Last edited by David the H.; 05-26-2008 at 08:50 AM.