puppy linux is pretty cool. i have it on a usb flash drive.
-from the site-
* Puppy will easily install to USB, Zip or hard drive media.
* Booting from CD, Puppy will load totally into RAM so that the CD drive is then free for other purposes.
* Booting from CD, Puppy can save everything back to the CD, no need for a hard drive.
* Booting from USB, Puppy will greatly minimise writes, to extend the life of Flash devices indefinitely.
* Puppy will be extremely friendly for Linux newbies.
* Puppy will boot up and run extraordinarily fast.
* Puppy will have all the applications needed for daily use.
* Puppy will just work, no hassles.
* Puppy will breathe new life into old PCs
http://www.goosee.com/puppy/