the poll isnt multiple choice :)
i use Windows XP pro on my home desktop, it also has a fedora 9 (dual boot)
My server which runs very loudly in my basement has: server 2003, server 2008 and fedora 9 running on it. I use it for development and software testing. At the moment I have hyper-v running on 2008 with centos and some other distros, mostly for learning purposes.
I do have a MAC G5 but I am selling that.
NOTE: i have swappable harddrives with different OSs on them, so its pretty easy to switch them out. Its a dell poweredge 2650