Knoppix is a Linux distribution that can be run from a cd-rom without having to be installed to a hard drive.
It can be installed to a hard drive if desired.
It is based off the Debian distribution.
It seems to be becoming quite popular.
The best English site for information about Knoppix seems to be http://www.knoppix.net
The official site for Knoppix is Klaus Knopper's website: http://www.knoppix.org