I'm tossing in another vote for Ubuntu.
It's friendly to new users but it's just another *nix like all the rest. It comes with synaptic package manager which makes finding and installing stuff a breeze.
The only time I don't use ubuntu is when i don't want a desktop, in that case i usually reach for netbsd or a minimal debian install.
Ubuntu on the macintosh is also very nice, i have 2 minis runing ubuntu. It's significantly faster than osx on the minis.
