So Microsoft is holding a sweepstakes for Xbox Live members in August.  All you need to do is download a demo of the arcade title released each Wednesday.  You can actually setup your 360 to automatically download these demos.  There will be a weekly winner that will get 4000 Microsoft points and there will be one grand prize winner and two 2nd prize winners.  The grand prize is a Xbox 360 Elite, 12 month subscription to Live and 100,000 Microsoft points.  The second prize is 12 months subscription to Live and 10,000 Microsoft points.

Now the five arcade games are Geometry Wars Evolved 2, Braid, Bionic Commando: Rearmed, Galaga Legions and Castle Crashers.  Now all five of these games look cool.  Geometry Wars Evolved 2 is a dual stick arcade style shooter.  Braid is a 2D side scrolling platformer.  Bionic Commando: Rearmed is the remake of the original 8 bit game.  Galaga Legions is a all new HD version of the original arcade shooter.  Castle Crashers is a new game that is a hack and slash game using all hand drawn art.  

Source:
Major Nelson - announcing-the-summer-of-arcade