I think that SET is my favorite Math Circle game. Not only is it challenging and fast-paced, but it has a great community aspect to it: players can join in during the middle of the game, or choose to play for a few minutes. A friend of mine found a SET app on her phone the other day, so we were playing while at a local Open-Mic night. A second friend saw it, played with us until she found a SET, and then returned to her group. It's that fast, and that fun.
Another way to play SET is to visit this website. There are 12 cards on the table and 6 SETs that you must find; every day there is a new puzzle. How long does it take you to find them all? My record is 42 seconds--can you beat that?