Windows 8.1 Pro x64 for my main desktop. The netbook that runs my mining hardware is running Debian 7.something at the moment, though it needs replacement soonish -- 4 GB of storage isn't enough. (IMG:[invalid] style_emoticons/default/smile.gif)
Fedora 21 *insert tipping joke here* Linus Torvalds approved! I was using Debian for the longest time, but I'm finding yum to be better than apt-get and the installer is really REALLY nice for a linux installer.