I'm a member of MSDN, so bleeding edge alpha/beta releases from Microsoft.....on a machine I don't put anything of value on.
Home Server: Windows Home Server
Desktop: Vista
Desktop #2: XP
Laptop #1: XP
Laptop #2: Vista
Test Machine: Vista with some alpha crap that doesn't like anyone or anything and if you move the mouse, you die.
Server: Unix is the flavor of the day.