Online Alternate Reality Games
I Love Bees: One of the most famous online ARGs of all time, created for the release of Halo 2.
Cloudmakers: The official website of the Cloudmakers, the original online ARG community created as a response to "The Beast."
Alternate Reality Games as Art Pieces
Desert Rain: A gallery installation based ARG by Blast Theory on the Gulf War.
World Without Oil: An ARG created to call attention to, spark dialogue about, plan for and engineer solutions to a possible near-future global oil shortage
Large-scale Alternate Reality Games
Cruel 2 B Kind: A city-wide ARG in the style of the game 'Assassins' with an interesting social twist.
Game Theory
Avant Game: Personal Website of Jane McGonigal, leading mind on ARG development and game theory.
Avant Game Blog: McGonigal's personal blog.
This Is Not A Game: An essay on ARG development
This Might Be A Game: dissertation on ARG development by McGonigal. This lady is seriously amazing.
What Wikipedia has to say on Alternate Reality Gaming: Surprisingly in-depth. Thanks, wikipedia.