OpenFeint
OpenFeint is an addon prepackaged in many iOS and Android games, that allows you to earn rewards and achievements, play with friends, challenge high-scorers, and build your own gaming profile.
Game Center
Game Center brings social gaming to OS X and iOS apps. With the Game Center app, you can play against friends and unknown foes ...
Scoreoid
Scoreoid, developed by game developers for game developers, is a non-restrictive, reliable and easy to use backend as a service.
Clay.io
Find games to play in your browser or directly from your phone!