Knoppix is one of the coolest Linux distros around, especially because of its amazing hardware recognition capabilities.
To have a full operating system running just from a CD without the need for even a hard disk was simply unimaginable to me before I learnt about Knoppix. I was simply blown away when I first used it.
Some cool stuff can be done with this kind of technology as well such as the Movix project which allows you to create eMoviX CDs which boots on your computer and automagically starts playing movies! You can use K3B to create eMoviX CDs using Linux.
Also, check out how Jeremy Zawodny has set up Knoppix on the home computer so that his parents can use it with ease.