Ubuntu Linux. I like how it’s free, current, has security updates that work well, based on Debian, and well-supported on x86, AMD64, and PowerPC.

A lot of the reason I want to switch to Linux is that I’m most familiar with it, and there is a lot that is different in Mac OS X, for example clicking on the window raising the whole application, and the “end” key not doing the same thing as it does in Linux/Windows.