Few of the main features...
*. Play almost all video and audio formats.
*. Skin-able interface (change background of the skin, etc).
*. Support a lot of remote controls.
*. Direct read from CD/DVD/HDD/USB drives, archieves (zip, rar, .iso), etc.
*. Album management.
*. Play-list support.
*. Fully HD video playback support.
*. Image viewing.
*. Automatically displays whether information fetched from the web.
*. Plugin and script support (which add various features to the media core app).
If you use Microsoft windows, Mac OSX and Apple TV then get it from here. If you have installed Ubuntu 10.10 then you can easily install it by using a PPA using the Terminal.
Open your terminal and issue the below command...
sudo add-apt-repository ppa:team-xbmc/ppa
sudo apt-get update
sudo apt-get install xbmc