Pink against green, and whoever completes a line first wins. A round takes barely a minute, so the game fits into any wait. Shlayolotl is made for the phone and runs on iPhone.
Three ways to play
Against the computer when you are on your own. Against someone sitting next to you. Or online.
Two boards
The classic board with three by three squares, and a bigger one with four by four.
No account, no sign-up. Whatever the game remembers stays on your device.