Ubuntu Linux > Windows 7 > Mac OS X.6 > Vista > XP
I'm currently running Ubuntu 9.10 on my laptop and desktop (desktop is virtualizing Win7).
Screenshots
http://tinypic.com/r/2ak0i9z/6
http://i49.tinypic.com/dq16io.jpg
Linux allows me to have a lot of flexibility over my system and frankly it's a saviour for programmers, built in Python/PHP and a majority of the GNU Compiler Collection. Ubuntu's gedit is a great programming editor IMO and it uses only a few MB of RAM on my system. Not to mention it's external tools plugin is awesome.