VLC is a free and open sourced very versatile video player.
I've used it ever since I can remember and I've never came across an audio or video file it hasn't been able to open and play (see the full list of supported files below). Aside from it's amazing playback capabilities, VLC can also be used as an encoder and a server to stream many codecs, file formats, as well as DVDs, VCDs and various streaming protocols. VLC, written by the VideoLAN project, is open source software licensed under a GNU General Public License, which means it is entirely free to download, use and share - even for commercial purposes. VLC is also one of the most platform-independent multimedia players - it is available for Microsoft Windows, Mac OS X, Linux, BeOS, Syllable, BSD, MorphOS, Solaris and Sharp Zaurus - and it has been translated to many different languages.