I almost don't want to re-post mine, because there are such cooler things... but hey, it's the first time I even tried the Challenge, and I think it came out great!
I always think it would be great to be able to whip a game out of my purse when my friend/DH and I are sitting in a restaurant or coffee house or park or airport, but most "travel" games are either too complicated to work well in tiny form (like Scrabble), too bulky to carry around, or too simple to be any real diversion (like tic-tac-toe or something). So, I made a travel game that *I* would actually want to play.
On the green side is an 8x8 grid, on which you can play Reversi (also known as Othello):http://en.wikipedia.org/wiki/Reversi
And on the orange side are two rows of 6 boxes each, where you can play Mancala:http://en.wikipedia.org/wiki/Mancala
You could also, I suppose, play checkers on the green side, since it is an 8x8 grid (and surely everyone can remember to play on the diagonal without there being 2 colors of square), or simplified versions of Pente or Go. The whole thing weighs less than a deck of cards, and can be thrown into a zipper baggie. It's also very durable, being made of washers and duct tape.
I laid out overlapping strips of duct tape to make a sheet green on one side and orange on the other. Then I measured and cut an 8-inch square, and drew the grids on with Sharpie. I used 64 washers, painted black on the flatter side, so that they are black on one side and silver on the other. I had bought a 99-cent can of spray paint, but the weather kept being cold, wet, and windy, so I ended up using no-prep brush-on metal paint, which was also very easy.
I'm already thinking of other games you could make this way! It was even easier to make than I thought it would be