If you seperate Ubuntu from Linux, then you should include all Linux variants. That my friend is a daunting task, as new vairiants are made everyday. I use Three operating systems on different machines. My server runs OpenBSD, my laptop Vista (waste of my time) and my Gaming Machine runs XP.
Edit:
Vista is a waste of money if you ask me. The OS is really intrusive when first installed. You double click on a program and it askes you if you want to run it. DUH, of course i want to run it.(I CLICKED ON IT!!) However, on the flip side it has somevery good firewall features.