Knoppix

What is it?
"KNOPPIX is a bootable Live system on CD or DVD, consisting of a representative collection of GNU/Linux software, automatic hardware detection, and support for many graphics cards, sound cards, SCSI and USB devices and other peripherals. KNOPPIX can be used as a productive Linux system for the desktop, educational CD, rescue system, or adapted and used as a platform for commercial software product demos. It is not necessary to install anything on a hard disk. Due to on-the-fly decompression, the CD can have up to 2 GB of executable software installed on it (over 8GB on the DVD "Maxi" edition)."

Knoppix Homepage

You should never leave the house without a copy.

Related: Knoppix3 | Gnoppix | Kanotix | Debian/Bootimages

External
CD that focuses on Incident Response & Forensics tools and which has been modified to specifically not touch the host computer and be forensically sound.
 * Knoppix STD Security Tools Distribution; a Linux-based Security Tool. Actually, it is a collection of hundreds if not thousands of open source security tools. It's a Live Linux Distro, which means it runs from a bootable CD in memory without changing the native operating system of the host computer. Its sole purpose in life is to put as many security tools at your disposal with as slick an interface as it can.
 * HELIX a customized distribution of the Knoppix Live Linux