Compilers , OSs, browsers, VMs, Databases, scientific experiments, high performance systems, HPC, video games, complex desktop applications, simulations.

In fact the question should be: what interesting projects don't use C++?

For some very subjective definition of intersecting of course.

This is obviously very subjective, but C++ is used in many interesting fields: robotics, graphics, signal/image processing, physics/game engine, ...

I'm currently doing some backend work, and I really miss it. I don't think I've done anything interesting algorithm-wise since I switched. Alas, the pay is usually much better in SaaS land.

What's your definition of interesting?