VLC Media Player
VLC Media Player can play any kind of video or audio file you throw at it. Compared to Windows Media Player, it is very lightweight. It is available for Windows, Mac, and Linux.
For people who download videos or those who just want something simpler to play their MP3s, VLC is a popular alternative.
Microsoft is promising a smaller version of Windows Media Player as an option when Windows 7 arrives in late 2009. Until then, VLC easily beats it.
VLC is also a useful alternative to the Quicktime Player on the Mac. There are several formats that VLC supports which the Quicktime Player does not by default.
You can run more than one media player on a computer, so give VLC a try.