Ubuntu Geek published a tutorial about installing Mplayer and Multimedia Codecs in Ubuntu 10.10

MPlayer is a movie and animation player that supports a wide range of codecs and file formats, including MPEG 1/2/4,DivX 3/4/5, Windows Media 7/8/9, RealAudio/Video up to 9, Quicktime 5/6, and Vivo 1/2. It has many MX/SSE (2)/3Dnow(Ex) optimized native audio and video codecs, but allows using XAnim’s and RealPlayer’s binary codec plugins, and Win32 codec DLLs.

  Install Mplayer and Multimedia Codecs in Ubuntu 10.10