ArcadeAid is a viral promo for the up-coming ‘Tron: Legacy’ film, and if the film is half as cool as this it will be a winner.
The link takes you to an enormous pixel-art image which has 167 retro game images within it. All (all???) you have to do is identify them. If you login you can save your progress & go back for more.
Give it a go, it is maddeningly hard!