XBMC is a fantastic cross platform (Windows, Mac, Linux) media centre. It's very powerful, has loads of features and is customisable and extendible via plug-ins.
XBMC is open source, it will always be free and people can help improve it.
"Currently XBMC can be used to play almost all popular audio and video formats around. It was designed for network playback, so you can stream your multimedia from anywhere in the house or directly from the internet using practically any protocol available. Use your media as-is: XBMC can play CDs and DVDs directly from the disk or image file, almost all popular archive formats from your hard drive, and even files inside ZIP and RAR archives. It will even scan all of your media and automatically create a personalized library complete with box covers, descriptions, and fanart. There are playlist and slideshow functions, a weather forecast feature and many audio visualizations. Once installed, your computer will become a fully functional multimedia jukebox."