Linux Live CDs: All the Linux with None of the Commitment has a great overview of Linux LiveCDs. It starts by explaining the different uses of LiveCDs, and overviews many of the popular LiveCDs available today.

And if that wasn’t enough, specialized live distros can run from business card-sized CDs, USB thumb drives, and some that are intended as rescue CDs for virus-ridden PCs. There are even live routers and firewalls in case you want to use an older PC as your main connection to the Internet.