Yes, that's right -- a distribution of Linux (Debian to be precise) which can boot from any Windows PC's bootable CD-ROM drive. It's called Knoppix, as it was packaged up by a German gent named Knopper, who worked out the CD-booting feature.
As a Mac OS X user (OS X runs on top of BSD, a variant of Unix something like Linux), I was surprised at how well it worked on my Dad's ThinkPad -- recognizing his hardware and even his ethernet network card with no configuration. In fact, Dad stopped by my house today today, so we plugged in a WaveLAN Silver Wi-Fi card from my old PowerBook and it worked flawlessly! It was almost like using a Macintosh. :-)
As I understand it, Knoppix specializes in the CD booting feature, which is great if you want to check out Linux without committing to a full installation. A CD bootable version Linux can also be a handy administrative tool, since all an administrator needs is a Knoppix CD to turn any Windows PC into a Linux box. If you're a Linux guru, you can even mess around with the bootable CD itself, saving your prefs on it and adding whatever apps you might want to have on the CD. Pretty slick, eh?
Knoppix is a full-featured distribution of Debian Linux, including KDE 3.1.1, the desktop windowing application (equivalent to the pretty part of Windows XP) that sits on top of the Linux base, or "kernel", Konqueror, a Linux web browser and OpenOffice, which can open and save Microsoft Office documents. There seemed to be many other apps installed that I didn't check out, however I can report that the windows-like KDE application looks pretty nice!
Knoppix is released as open source software, so once you buy a master install CD you can make as many copies as you like and install Knoppix on as many computers as you like. The CD is only $6 and Knoppix can even be had for free, if you fancy doing a download over your cable modem connection.
So, if you've ever been curious about what the heck this Linux thingee is like, check out Knoppix. Linux couldn't be any easier or more painless! :-)