I am a freelance C++/Qt developer.
I am a C++ fan-boy and I am a fan of anything which is open-source, and support and follow it religiously.
Thus my primary OS is Ubuntu Linux and for the same reason I chose Qt as primary tool for all my development needs.
As such, I (have to) develop mainly for Windows OS (due to it's large market share), but I also develop for (Ubuntu) Linux and Mac OS as and when needed. For all the apps I use personally, I develop and use in Ubuntu Linux.