Some examples are:
* LLVM: you can build a programming language that compiles to LLVM IR — and get all the optimizations for free.
* SAT solvers — convert your NP-hard problem to SAT and run a SAT solver on it.
* Boehm(-Demers-Weiser) garbage collector — collect garbage without actually explicitly writing any garbage collecting logic.
Oh, and the computers all that stuff and the the things you build by tying them together run on.
Just for starters.