While alternate reality games are a fairly new thing, we can find many antecedents to them in other media.
Any additions, after this list? I will update.
What a fun syllabus this would be, if we include games!
Very male sequence, though, so far. Women do create ARGs. Is that a historical change, or am I missing female-authored antecedents?
Antecedents to alternate reality games
(updated February 17, 2006)
Assassin, also called Killer. Game played by people usually in some educational institution, where they try to kill each other according to a pre-established scheme. Assassin must occur within these real, non-game settings. Not too narrative, as Tony notes.
Iain Banks, The Player of Games (1989). Science fiction novel, focusing on a world where a game structures culture and society.
David Fincher, The Game (1997). Film. Protagonist signs up to play a game which weaves itself into his life, changing him for the better.
William Gibson, Pattern Recognition (2003). Science fiction novel. Characters follow The Footage, a mysterious, distributed, film/video microcontent project. The Footage isn't published to a single venue, but appears through discussion fora. The reality of what it shows isn't clear.
How to host a murder parties.
Ray Johnson (1927-1995). Artist, whose work wove into audiences' minds and lives, unsettling (among other things) the boundary between life and art. Cf the film How To Draw a Bunny (2002). Performance art in general is a rich proto-ARG vein.
Live-action role-playing games (LARPs). Not intrinsically ARGlike, but become so when played within a non-game setting (i.e., a town, school, party).
Luigi Serafini, Codex Seraphinianus (1970s). A mysterious, gorgeous book portraying a fantastic world in a coded language.