There's a topic like this everywhere else you look so I think it would be good to have here.
The concept is simple I'll post a screenshot of a game, and the next poster has to guess the game. Then he posts a screenshot, and the next person has to guess it, and so on and so forth.
There's a few rules I think should be followed. For one, I think only screenshots from released games should be posted. No use posting a screenshot from a game in the alpha stage and not released til 2012. The other rule would be that if you find a screenshot that you want, then don't hotlink the image. If only because it's then too easy for someone to right-click on the image, view 'Properties' and the game name might be included in the file name. Best bet is to save the image you want, rename it and then upload through ImageShack.
I'll start with the easiest one I could possibly think of to get the ball rolling: