This is a tic-tac-toe game written in JavaScript and HTML 5 with an AI that's pretty much unbeatable. You can also turn off the AI and play with a friend if you like.
I wrote this in 2012 and the AI is pretty much a brute force solution. I could rewrite it and make it a lot nicer (the code certainly needs it), but I'd rather keep it in its current state for nostalgia's sake.
If you manage to beat the AI, let me know on Twitter. Be sure to tell me how you beat it, so I can make it better ;)