A free first person shooter with good online play. If you like Quake Arena you'll love this one. I had fun with it for hours (that's a lot for me with a FPS - I get bored easily with them). The graphics are nice and the game play is fast - have a blast! There are versions for Windows, Linux, and OSX.
http://icculus.org/alienarena/rpa/

SpaceSniffer is another filespace visualization program that uses the TreeMap concept to show how the hard disk on any Windows XP, Vista, Windows 2000 computer is arranged. Since its freeware/donationware, you can head over to http://www.uderzo.it/main_products/space_sniffer/index.html to download it and check it out. I've checked out the examples that come with the TreeMap component while playing around with Java and this one is better. The zoom feature is the feature that sold me on it (the feature makes working with it a little more enjoyable than the other programs that use the TreeMap concept). This program is a portable program which means that you can run it off of your flash drive without installing anything.
I've had problems with the zooming feature however. It resets itself to the standard interface on its own. If that happens to you just restart the program and it works OK afterwards. Even thoug I've had problems with certain features of the program it does its job well.

Getting this beta download will give you a glimpse of the new Windows not yet released. Use IE for the download though as the applet in Firefox will give you a problem if you try to retry.

The download page:

http://www.microsoft.com/windows/windows-7/beta-download.aspx