I am a trained Linux admin so I use a ton of OSes.
Main - CentOS 6.5 x86_64 Full and Minimal
SmartOS - it sucks
Sun OS and Solaris 6; and 11
Windows XP
Windows 7
Windows 8
Windows 98 (exclusively for roller coaster tycoon)
Ubuntu 11.4 for shenmue tool development
BSD on my PS3
Mac OSX hackintosh on my laptop
Kali Linux for professional pen testing