Well it depends on which machine we are talking.....
I have XP Pro on my main box
BeOS on another one
Win98 on another one
Longhorn on another one
Red Hat on another one
XP Pro on 2 more......
Also have a mac laptop
That is me and my rommate's comps. So I guess you could say a little bit of all of them.
I added a couple of options to your poll. I use Windows 2000 Advanced Server so I added a Windows 2000 Server option. Windows 2000 SP3/SP4 option reallly referred to workstation I think.
There is also an "other" option for any people running FreeBSD, Solaris, etc as their main OS.
XP almost always now. win2k previous, and win98se occasionally.
Also tried ME for a while but not impressed. Win9x kernel sucks big time, once you've tried any NT based OS (XP or 2k) so I would'nt go back (ever noticed how the mouse freezes in win9x OS when some tasks are running?).
I run WinXP on my main home machine, Win2K Server on my main work machine, Mac OS X 10.2.4 (Jaguar) on my main work Mac, and Mac OS X Server 10.1.4 on my home Macs except the one running Mandrake PPC.
XP PRO on both systems now as Longhorn for me - when ever i played a DVD out to my TV - it said my TV was not working - but i could use it normally and watch avi's and such fine.