In Misère Tic-tac-toe, getting three-in-a-row means you lose. I can't wait to unleash this game on my kids at our next dinner out.
Here is a wonderfully-named app version: Notakto
Here is a scientific paper on the mathematics of the game: "The Secrets of Notakto: Winning at X-onlyTic-Tac-Toe" by Thane E. Plambeck, Greg Whitehead
And below is part 2 of the Numberphile video. Note from the video description:
Correction from Thane: The configuration with two X's, one in a corner and one in the middle, is a "b" and not a "b^2". It's at 5:24 in the video.
(via Cliff Pickover)