Poll: What OS do you use?

What operating system do you use on a day in, day out basis?

I want to know your optimal environment, as most of us use at least two OSes weekly or daily.

I'm going to stick to just one option for Linux; feel free to elaborate on distro/wm/environment in the comments.

(FYI: It's been a year+ since there the last poll of this nature and traffic has doubled since then, so I think it's allowable :-) )