Ubuntu - We all use it in one form or another, it's sister project xubuntu is pretty sweet for old laptops or if you want to get more bang out of your box.

Paranoid Linux - A great distro for any activist to have, especially those living under repressive governments, this distro enforces encryption at every step of the way and even injects random traffic into your data stream!
 

Damn Small Linux - Necessary for every tool kit, at only 50 mbs how can you go wrong?

Backtrack - a full security toolkit, very helpful and should be kept on hand. Use with with Damn Vulnerable Linux for maximum fun.

Windows FLP - NOT LINUX. but also not well known, it installs a really simple windows machine with driver support and internet explorer. can fit in under a gig of hard drive space and has no frills. When you really need windows, this is your best bet. Get It Here