MPlayer

MPlayer is used to playback audio and video, either from files, or delivered live through the net.

Its one of those softwares that you may never have heard of but once you have used it, you don't know how you could do without it. It plays almost everything, including CDs, DVDs, audio files, VCD, video files, net radio, streaming video. Whatever there is to play, it plays. Its also a lovely software because you can install and run it on every operating system. The look and feel, and navigation, is a little different on each operating system but it essentially does the same thing. If you are used to using it on Linux it doesn't take much to figure out how it works on OSX or Windows. For that reason its an excellent choice for installing in operating systems you might not be used to, as well as using on your own computer.

MPlayer also allows you to manage playlists of material, so you can use it for playing back your music library, and it handles full screen video very well. So watching DVDs or DIVX (etc) with it is a pleasure.

If you are using Ubuntu as an operating system it is much more powerful than the default media player (Totem), and has a nicer interface, but it can be a little bit more tricky to configure. If you have patience then stick with it, its well worth it.