Disinfect the Core
Disinfect the Core is a Shockwave game by The Jab that can be frustratingly difficult. Similar to games like Go and Othello, this game is about turning all the tiles (or I suppose 'nodes' in this case) to your color. You play as green and you may only click on a green node, doing so turns the node's arrow clockwise. Then, beginning with the first node the arrow points to and continuing along the circuit, each node is flipped to your color until there are no more nodes on that circuit. It's very simple to pick up and play, but the possibilities for each move become enormous.
With so many combinations to examine as a human, against a computer AI able to process many hundreds of thousands instructions a second, the odds are definitely stacked against you. However, the challenge is fun if you put a little time into examining how each move affects the play. There are some strategies that become apparent, like breaking up the train so that one move by the computer doesn't completely annihilate you. The game starts out by saying "You cannot win," but it sounds more like a taunt to me.