Some time ago, I saw that CERN people had their own clang tree with a few addons, most notable one being the C++ REPL (C++ interpreter) called cling.
Now we had a presentation by Pere Mato from CERN who talked about their ROOT data analysis framework. It seems like a really nice and powerful piece of software.
The software is around 50 million lines of code, mostly C++. Some of it is python, but it is only used for quick-and-dirty testing of new ideas.
Another cool thing is that they use Qt for writing UIs, and that they also run Linux and KDE Plasma.