I learned Prolog and logic and constraint programming in general. Not only did I learn the Prolog language and some of it's dialects for constraint programming and fuzzy logic as a part of a logic programming course, I also wrote an interpreter for a Prolog-like programming language in Haskell as a part of an AI course.

Prolog and Logic programming in general are really worth learning. They will definitely widen your horizons when it comes to programming. Implementing a logic programming system will also teach some basic pattern matching and unification algorithms and applying them.