Yu-Gi-Oh is fun! You can download these abandonware yugi titles and build your own deck and play against the AI:

Someone also wrote a save handler for the games so it saves your deck and won cards.

I know next to nothing about the game outside the manga which I read a little bit in some shonen jump issues I had back in high school. It's super easy to play, much easier than magic the gathering I've found, but like all good games has depth and is hard to master.