We develop trading software exclusively on Windows, and I use it for internal tools and our testing infrastructure. There are a few C extensions written to wrap our libraries, a lot of integration, regression, performance, and load libraries, and a bunch of dev tools to setup environments with our dependencies and such.

I introduced Python to the company in 2006 after writing a few very quick and dead simple test tools, and it seemed to pick up steam once our QA organization was looking for a way to beef up automation efforts.