Media Player Classic Home Cinema is a rewritten version of Windows Media Player 6 which was then open sourced and released under the GNU GPL. The interface is similar to the MS original but is a little more up-to-date, XP friendly, comes with some more advanced features, and doesn't have the annoyances of the original. But probably the best thing about this player is the range of formats it supports. It'll play just about anything you throw at it, whether it's an MP3, an Ogg, a WMA, even a QuickTime file. Media Player Classic - Home Cinema application was designed to be a Media Player Classic but for home cinema usage.
Thanks to MKVlover for the update.