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Several people have mentioned specific brands of Live CD OSes but I think the category should be placed somewhere on the list. It seems several distros came out with Live CDs this year.
Live CDs provide a tool for system admins and network specialist. They are also are fun way to learn and experiment with something new without destroying the main working system.
In a few years there will be Live DVD OSes.
A couple of brands to try. Damn Small Linux, Knoppix, Gnoppix, Knoppix-STD, SuSE, and I think Mandrake has one.