For those of you who do not know already an ARG, or Alternate Reality Game, is something that crosses many social, technological, and physical boundaries as a puzzle presented by one group for another group to solve. In this case it was a pre-release scavenger hunt presented by Valve (the makers of Portal) to promote the newly released Portal 2. The media used for this ARG was almost limitless, with ridiculous methods used such as HEX translation, Morse Code, pictures containing letter cyphers hidden in downloadable game content unlocked by beating certain levels with certain time or dificulty constraints. The ARG lasted 19 days (leading up to the release of Portal 2 this morning) and was based around a group of 13 games sold by Valve as a bundle called the Potato Sack. The final step involved booting up GlaDOS using cpu power generated by Steam users playing these games, with potatos earned from a number of sources eventually overclocking the system and launching Portal 2 at midnight instead of 7am PST. The chronolog is definitely worth a look through, some of the puzzles were absolutely insane to figure out and required the effort of a large amount of the steam community.
Links to the GlaDOS reboot cite: http://www.aperturescience.com/glados@home/
Chronolog of the entire ARG: http://valvearg.com/wiki/Valve_ARG_Wiki