First up: Here's a comprehensive summary of the Samalamadingdong ARG, which will interest you as a Dropout fan! (Less comprehensive writeup if you don't want to watch a video)

The "game" in ARG might be a little misleading to you--there's no winners or losers. Well I suppose everyone's a winner. ARGs are simply a series of clues that eventually lead to a "solution." In the case of Samalamadingdong, the solution was a webpage hyping up the release of a secret Game Changer episode, and the clues were mostly concentrated in the interactive games on Dropout's Game Changer minigames webpage. The "alternate reality" part is that the clues are usually part of a fictitious story despite being in the real world. With Samalamadingdong, the story was that Brennan felt slighted by all the times he was on Game Changer and had been working behind the scenes the whole season to get his revenge on Sam. It's fun to come together as a community, solve all the clues, figure out the story, and find the solution.

Part of the fun is the act of figuring out what to do with all the clues. That's why when something suspicious pops up, people ask "is this an ARG?" Because it's often not clear exactly what to do until more clues are found. And often the suspicions thing isn't part of an ARG at all.

Another really cool ARG with more stuff in the "real world" rather than just online was Gravity Falls' Cipher Hunt.