Knoppix

Description: This Linux distribution runs entirely from a bootable CD. Data on the CD is uncompressed on the fly, allowing them to fit 2 GB worth of system and programs on one CD, including a complete X server, KDE and Gnome, and large packages like OpenOffice and The Gimp. Since it runs solely off the CD, Knoppix makes an excellent portable Linux demo or system rescue disk, but its completeness makes it a good general purpose distribution as well.

This is a security concern---^ pretty neat too.