Amnews.com has an article about Puppy Linux. I’m not familiar with this source, but it looks like LiveCDs are becoming more and more mainstream.
Puppy can be run from a live CD or installed on a hard drive, flash drive or ZIP drive. When booting from a CD, Puppy does not require a hard drive because it can save everything back to your CD. In order to do this, however, a CD burner is required.