I recently happened upon Knoppix and was totally blown away. It’s a Linux distribution (Debian-based) that runs completely off of a CD. You don’t have to install anything. You can pop it into your Windows machine’s CD-ROM drive, tell your computer to boot off the CD, and Knoppix loads. It auto-detects your hardware (on my laptop, it even autodetected the wireless card and I was running wireless instantly) and you are up and running in a KDE 3.1 environment.
For people who are interested in Linux but don’t want to go through the aggravation (“fun”) of getting it installed and getting peripherals working just to try it out, this really is a great program. You can go back to Windows just by rebooting your PC and taking out the CD. Piece of cake.