Grantoo!

Game Center brings social gaming to OS X and iOS apps. With the Game Center app, you can play against friends and unknown foes anywhere in the world. Invite someone to join, then get a game going. Or go up against people you don't know, from anywhere in the world, in a multiplayer game.

Scoreoid, developed by game developers for game developers, is a non-restrictive, reliable and easy to use backend as a service.

Plus+ is an addon for iOS games that connects you to a community of gamers like you.