Windows XP Professional Edition
Windows Server 2003 Enterprise Edition (360 Day Trial :P)
Fedora Core 4
Fedora Core 4 x86_64 (64-Bit Version)
I use Windows XP Pro the most. Then Win2k3. Then Fedora Core 32-Bit. I'm not to happy with the 64-bit edition though. It goes noticably faster, but a lot of common software does not work with it because it does not have a built-in 32-bit application layer like Windows x64 does. For example, the Flash Player does not work in Firefox due to this. But I must say, all of the software that comes with it works perfectly and would be great for a server.
I'll buy Vista if they make a 64-bit version of it. That would be cool .