ARGNet It's hard to say where to start with this site. It does report on a variety of ARGs, games and transmedia projects. Could this site inform an ARG you or your students designed?
So let's start thinking about mysteries, puzzles and scavenger hunts. You can blend elements of all of these into an ARG, IF or any problem based learning assignment.
To put you in the right frame mind, I would suggest watching National Treasure 2004 with the actor Nicholas Cage or at least taking a look at this National Treasure Book of Secrets "hunt" sponsored by Disney. Mysteries Tuesday 7- 8 pm MST (Denver) DIY: Mystery with Vasili Giannoutsos
"Who? What? Where? When? Why? The 5 Ws of the Mystery World and soon you will know the ins and outs of these burning questions. We will be going over a brief history of Mystery, showing the shakers and movers of Mystery in literature, television and film and how this translates to games. We will be talking about the kinds of Mysteries out there, and yes there are different kinds. How do we start making one? Let's talk about it! Through an audience chosen scenario we will go about the different Questions and processes to answering these questions and making Mysteries a reality for you."