I wouldn't say Operating Systems actually. The Linux & Windows kernels are mostly C, with only userland stuff being C++ (granted, drivers could be written in anything). I'm not sure about Mach (OS X), but I wouldn't be surprised if it were mostly C as well.

C++ filled this wonderful nitch of "Object Oriented, but fast enough for dinky computers in the late-80s & early-90s". Accordingly, anything written in the 90s had a good chance of being written in C++; applications, drivers, games, whatever.

Today, new development seems to be done mostly in managed (JVM/.NET) languages. Not that there isn't any new C++ development; games, in particular, are still performance limited enough to use C++ in many cases.

In short:

Drivers;Games;LEGACY CODE