FPGAs & CPLDs. Of course those aren't really CPUs to be programmed with software, they're another level of abstraction down. But they're pretty common and hard

FPGAs & CPLDs. Of course those aren't really CPUs to be programmed with software, they're another level of abstraction down. But they're pretty common and hardware description languages are vastly different from C. The inherent massive parallelism of FPGAs & the resulting combinatorial-by-default (sequential only when explicitly declared) languages requires a very different way of thinking.