Knoppix is a linux distribution with quite a few interesting properties.

• It runs entirely from the CD, without touch the hard drive. Just put the CD in and reboot. Providing your PC can boot from the CD drive, and Knoppix can locate hardware drivers, linux comes up.
• A compressed file system fits a lot of software on the CD. It's a very full featured KDE based system, with browser, email, office tools, the gimp, all sorts of goodies.
• It has good hardware detection, so this avoids hunting for drivers on the web.
• Once you have the system running, you can install it on your hard drive. Knoppix is based on Debian, so it solves a long running problem with Debian - the painful installation process.

There is also a cottage industry of people doing variations on the Knoppix theme, creating different CD images for different purposes. So you can choose a linux environment to your taste, or even set up your own. A good example of expanding this notion is Morphix, which is exploring a modular way to develop such custom CDs, and currently offers several packages for different purposes.