You could expand this category to an AI for a number of similar games (like checkers and reversi) as well. I've written a double dummy solver for bridge--a program that computes the optimal result of the play of a bridge hand--as a side project and I think it's also a very good choice. A lot of the same tools as chess engine development apply: alpha-beta and the myriad variant search algorithms like MTD(f), transposition tables, heuristics like the killer move heuristic or the countermove heuristic. But you also need to apply some domain specific heuristics and tools, and since the whole thing is performance critical you need to worry about low level optimizations and good multithreading. In the bridge case it's not at all trivial to get something that can even finish at all in a reasonable timespan.