I have been carrying this portable application on my thumbdrives for quite a while now, because it will play most any format and I sometimes at other PCs than my own. When I find a music or video file that does not play on that other PC, usually it will play once I load it into this portable application.
I also have the regular install version on my desktop. It is set up to be the default media player. The install version can be found at http://www.videolan.org/vlc/. It is currently at version 1.1.0, and I updated my desktop install also.
Visit their site for all the available features and supported platforms. I know it does a lot more than just function as a media player, but I do not use any of those features myself, so can not attest to their usefulness.
If anyone uses the advanced features of this portable application, or even the full install version, please let me know by leaving a comment. Maybe you could present your findings to the SIG, and teach us all how to use this more effectively.