Kernel hacking, drivers, virtualization, FPGAs/ASICs, compilers/linkers/toolchains, bootloaders, firmware, debuggers, databases, shells, process managers, etc.

Kernel hacking, drivers, virtualization, FPGAs/ASICs, compilers/linkers/toolchains, bootloaders, firmware, debuggers, databases, shells, process managers, etc. etc. There's plenty. Generally if you don't have to worry too intimately about VMM semantics, you probably aren't doing low-level.