Koppix is a kick ass Linux distribution I use regularly to troubleshoot problems, diagnose bottlenecks, data recovery, showing off linux, or just to have a linux installation that works in an unfamiliar or hostile enviroment. It generally is run from a disc and it runs in memory so you don't have to install it on the computer you're using; this makes it great for forensics or data recovery.
There are a number of spins off the Knoppix disc that have specific purposes and goals, these work well too. You can even make your own knoppix disc to include software that you want to have on your live disc.
Here's the official description and some more information about Knoppix.
The Knoppix CD and DVD include recent Linux software and desktop environments. The DVD includes programs such as OpenOffice.org, Abiword, The Gimp, Konqueror, Mozilla, Apache, PHP, MySQL and hundreds of other quality open source programs. - knoppix.net