A storehouse of pencil and paper games

Here's a nicely organized collection of games that one can play with only pencil and paper. Really handy to learn a couple of these for entertaining kids on the fly.

http://www.papg.com/


I also found this interesting tic-tac-toe variation, ultimate tic-tac-toe:
http://mathwithbaddrawings.com/2013/06/16/ultimate-tic-tac-toe/

...which has a smaller board inside each square of a larger one. The kicker is, you must make your move on the small board corresponding to your opponents last move.