Knoppix is a wonderful idea. You can boot a full-fledged Linux with KDE3 desktop and many useful programs (about 2GB worth total) running completely off the CD. You can have a completely usable OS without a harddrive at all.
It autoconfigures all your hardware (video, monitor, sound, network, USB, etc.) with no intervention needed on your part. The load time is very nice.
Here's one of the biggest uses of it that I've come across:
Once it's up and running, it gives you an icon on your desktop for each hard drive partition it found and lets you browse the filesystems. I think it defaults to read-only mode, but all you have to do is right-click and select read-write mode. Get this: It will browse ext2, FAT16, FAT32, and NTFS (e.g., Windows 2000).
It's a great item to have in your toolkit.