One of the main features of ARGs, as anyone interested in them will tell you, is that they deal with “collective intelligence” (or, as JC Hutchins puts it, “bunches of brains”). The idea is to get lots of people together, thinking about the content and contributing their ideas. Very often the players create a place for those ideas to be recorded. You could just use the Unfiction Forums, of course, but it may be nice to have your own place.
Aside from organizing information about an ongoing game, these websites also serve as archives when the games are finished. The experience, the process of playing the game is often captured. In fact, there are also sites dedicated to archiving information about ARGs past (although, curiously, none of them ever seemed to get past the planning stage).
Here are some sites organizing information about ongoing and, possibly, past games:
A couple intended as “ARG archives”: