Windows Server 2003 Workstation
I realize I'm waaaay behind the crowd on this one, but I just moved to a different machine here at work and I setup Win2K3 as my development machine and so far I really like it. I've got it so it looks and works just like XP, except now I have IIS 6.0 (that and it's more secure of course ;)).
I already knew most of the stuff to change to get it more like a workstation, but this one helped me get everything.
Only thing I've found that doesn't work is my mp3 player (Archos Jukebox Recorder 20) doesn't show up as a hard drive, other than that, everything works great! :)