When bing makes it a game, I'll list theirs, too...

I am a little disappointed, since Google dropped me as an ad publisher. But praise is due: they managed to make a game out of improving their picture tagging on their image search. You can pair up with a partner, competing to see how quickly you can come up with the same word when looking at a given picture.
Note: when their competitors make a game out of helping them, i'll be glad to play...
Subtlety is not one of my strengths