Powerful media player and entertainment hub
Plex, (based on the cross-platform XBMC), is a free and open source (GPL) home theater entertainment hub and media-player, designed to cater for all your viewing needs.
Plex can truly play or display just about anything from videos to slideshows and even weather forecasts. There is also a host of third-party plugins which can expand the functionality of Plex even further. Plex is an impressive application but it should be noted from the start that it is quite overwhelming. You'll have to spend quite a while just reading through the help guide to understand how to use it.
Plex adds many more features that were found in XBMC including the Plex Media Server, online video (including Hulu, TED, CNET, Revision3), full iTunes Music support, iTunes DRM Playback, iTunes Visualizers, and Sparkle Updates. Controlling Plex is a pain though. Due to its intended use on big screens, you can only control it via the keyboard or the Mac remote. You have to load in all of your music, videos and other media because it doesn't perform any kind of scan and it won't play all audio formats. On the plus side, Plex looks great - it looks like Front Row on steroids.
Plex is a great looking home theater app, with impressive video capabilities. It is let down somewhat by difficult setup and controls, as well as poor music library support.