Knoppix is a free, widely-used Live Linux CD distribution, based on Debian GNU/Linux. It bills itself as from zero to Linux in five minutes,
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As is true for many Linux distributions, the Knoppix community contributes to documentation for it. You can find lots of help at the Knoppix wiki. Knoppix has also given rise to many knock-off versions that also run from a CD or USB key. The one that has really caught my eye is SliTaz, which can sit on your USB key at only 24.8MB, and is fully open source. You dont get anywhere near as many applications as Knoppix gives you, but you get Firefox, an FTP client and many other useful applications for free.