I read a post today on Lifehacker about a web site called OSvids.com that lets you preview operating systems, including Linux distros. Their front page currently has a video demonstrating Xubuntu — a lightweight version of Ubuntu.
I recommend checking out their video called Dapper XGL. "Dapper" is the name of the latest Ubuntu release, and "XGL" is the new advanced graphics system for the Linux desktop.
If the .ogg videos on OSvids.com won't play in your media player, you can download the free VLC media player. There is also a Portable version of VLC here