The Maw. Now, so far, the only Summer of Arcade title released has been Bastion.
The Maw was a critically acclaimed action-puzzle game released two and a half years ago back in January 2009, in which the gamer played an alien who was accompanied by a ravenous purple, well, maw which could eat objects and creatures and take on the properties of the thing it eats, such as breathing fire, flying, or simply growing in size. The Maw was developed by Twisted Pixel, whose latest release is Ms. Splosion Man.
Check out the official Summer of Arcade game site.