VLC Media Player

VLC Media Player is, as the name suggests, a media player. It is very lightweight and not very taxing on your computers resources, it can play virtually any file (audio or video) that you throw at it (I have been using VLC for many years now and have yet to come across a file it cant play). I really reccomend you try VLC, and why not, it's free and works on most platforms including Windows, Mac OS X, Ubuntu, Android, Debian etc.

(Download at http://www.videolan.org/vlc/)