In addition to being the theme of the 2008 Boston Celtics, Ubuntu is a flavor of linux – an open source, free operating system. They have a 6 month release cycle, so a new version comes out twice per year. A new version was just released today – you can check it out here:
If you want to try it out, you can run it as a live disk, meaning you can run it directly off of a CD or USB stick without affecting your current operating system (though with limited functionality).