If you like abstract games, especially chess variants, you should enjoy Zillions of Games. It’s quite an amazing piece of software; a piece of AI technology that I actually would have not thought possible.
Games are defined using a rules file (written in a Lisp-like language), and then a generalized alpha-beta search engine is unleashed. Basically, it works best for Chess and its many variants, but it plays a really surprising number of games very well. It comes with over 350 games and puzzles, and you can download thousands more.
It’s not that hard to modify existing rules files to define new games; my son was interested in having an opponent for a game he invented and we were able to modify an existing rules file to play his game in less than an hour–and it played very well.
Naturally, it doesn’t play all games as well as humans, but it plays so many games so well (with adjustable levels to make a fair fight if it’s too good) that one can’t complain. Unfortunately, it’s only available for Windows computers.