I don't know if it is the #1 most useful thing, but graph theory has to be up there in the top 5. I've used it to: - Figure out which parts of a process can

I don't know if it is the #1 most useful thing, but graph theory has to be up there in the top 5. I've used it to: - Figure out which parts of a process can be safely parallelized - Represent a complex process as a state machine/graph, and then achieve high test coverage by writing a test for each edge. - Write code analysis tools - Implement various phases of a compiler