I use the following, most of them pretty frequently:
OS X 10.5.5 (and OS X server)
Windows XP Professional SP2 and sometimes SP3
Windows Server 2k3
SuSe Enterprise Linux
Debian (or Ubuntu)
Vista Ultimate 64
As far as my main use goes, I would say that I use my Mac (10.5.5) and my Vista box the most. With the Vista box mainly being used for internet and gaming. Everything else I typically do on my mac. I am highly disappointed in Vista and think it runs like crap compared to my PC specs, and I get memory leaks and all kinds of stuff after periods of usage on it.
Linux is an OS that most people try and leave behind. It will take time to get really proficient at using it, possibly a year or two. It is different than Windows and 99% of people won't take the time to fully understand how it works. Then you have the people who just boot off live distros and play with it every now and then and say it is OK or they say they like it but they use Windows for over 95% of their computer needs.
XP - is the best Windows OS out right now IMO. If I weren't so lazy when it comes to my own personal machines I would probably toss XP Pro back on my gaming PC.