Tictac.io was almost the first game I covered from the treehouse. It's noughts and crosses (aka "tic tac toe" if you don't acknowledge that the ground is a floor) online, except in realtime, and you can pick up and move the pieces after you place them. It's bloody great.
We weren't able to get it working properly here in the office. I've no idea why and it's a real shame, because when I played it at home with my friend Deborah, it was chaos. There are many games that are tense but exhausting and faintly unpleasant for me. Tictacdoteyeoh's short games, frantic music and slow movement made for a fierce and exciting tension that invariably ended in both us of cackling with laughter and relief. We both figured out a handful of strategies at about the same time, and quickly copied ones we hadn't thought of from each other.
The best decision it makes is letting you move your opponent's pieces. First we only moved our own, then we started moving each other's. Then she realised she could second guess my plans and stack pieces under my hand to trick me into scoring for her instead. Then I began to open every match by moving her pieces into distant corners of the screen to waste her time. Then she started doing the same and I switched up tactics to start while she was distracted. If it had a motto it'd be "oh you COW".
Tictac.io can be played for free on its own site.