Anything in embedded systems. I write code for high performance satellite terminal (ground side) and also point-to-point radio systems (ultra-low latency, millimetre wave stuff). 90% of the code I write is in C, about 60% of the time on microcontrollers with <256KB of RAM, and the rest in a cut-down Linux running on an ARM Cortex A8.

I'd actually prefer to use C++ for the Linux stuff (std::string, smart pointers and STL containers would save me so much time), but we don't have the C++ standard library in our custom distro.