The Lost Ring: Alternate Reality Game based around the Olympics

From The Lost Ring Wiki:
The Find The Lost Ring ARG is a project being run by a group of teams well-experienced in the ARG genre, including Jane McGonigal (of World Without Oil), AKQA (of Iris), sponsored by none other than McDonald's, and combined with the backing of the International Olympic Committee itself. Suffice to say, it promises to be a grand scale game, planning to unite communities from around the globe beyond any language boundaries.

Its ultimate purpose and outcome is still unknown, but discovery is the fun of the game.

Official Site: www.thelostring.com