Linux, Linux, Linux (except when I have to use windows for work)
I'm surprised there aren't more Linux users here, but I guess its nerdy, whereas mac is cool (although they share some similarities, namely a unix background)
I like being able to build computers out of the trash, and run free software on them that's not obsolete,and spend $6 on the whole thing(my home server)
The open source software culture is probably what got me into fixed-gear cycling in the first place