Canonical have created an impressive online tour of the latest Ubuntu build. It's not your common-or-garden slideshow of features - it's a very nicely put together simulated desktop experience in HTML5, which should run in any decent up-to-date browser.
If you want to get really recursive, you can even do the tour within the Firefox browser from within the tour, ad infinitum.
I would tell you more, but that would ruin the fun. So if you've been meaning to try out Ubuntu but haven't had the time to install it on a machine (virtual or otherwise), head over to http://www.ubuntu.com/tour/en/ and have a play!
Scribbled by Tom