Here's something nice, but also depressing. I started playing The Evolution of Trust [official site], a short browser-based game, expecting it to show me why trusting other people is a good thing. Ten minutes in, it's taught me that I need to cheat more.
It has you playing a quick 'Game of Trust'. If you stick a coin into a machine, the person at the other side gets three coins, and vice versa. So, should you co-operate and play the slot, or cheat, withhold your money and hope the sucker on the other side is feeling generous?
It's really a teaching tool for learning the (very) basics of game theory. It talks you through your options, takes you through different scenarios and discusses how trust can evolve over time, hence the name. In the coin game, it turns out that always cheating is the best option if there's less than five rounds, but if there's more the best way forward is to copy your opponent's last action. I won't explain all the reasoning and mechanics behind it – if you've got a spare half an hour then give it a go and find out for yourself.
It's well-presented with scribble-y hand drawn characters and feeds my love of beautiful infographics. The music is a little annoying if you've got it running in the background, so maybe mute the tab if you get pulled away.
By the way, if you want some competition, I managed to get 27 out of 49 in the test, which is about five minutes in. Let me know how you fared!