I've gotten a lot of use out of group theory and lattice theory, and particularly semigroups and semilattices. What most programmers don't realize is that design patterns (a la Gang of Four), when they aren't working around a language's quirks, are implementing some algebra.
You know, I don't have any. It's something that I've thought about writing about for years now, but haven't gotten around to doing, and I'm unaware of anything else written about it.
