I don't know much about ARG. At first sight, maybe because I'm kind of lazy as a user or too pragmatic as a marketer, I see them complicated, requiring too much of an effort to be really engaging. But I don't want to bias you, nor to pan a new trend in interactive marketing that I still haven't explored in full.
I assume you are as ignorant as me about ARGs, so I start from the beginning, with the definition, and Wikipedia has the answer: an alternate reality game (ARG) is an interactive narrative that uses the real world as a platform, often involving multiple media and game elements, to tell a story that may be affected by participants' ideas or actions.
And now let's go into two recent examples. The first one launched last week in the UK, it's called We tell stories, and it's run by publishing giant Penguin. As explained on NMA, for six weeks, a story a week will be added to wetellstories.co.uk. Each of the stories, written by top authors including Mohsin Hamid and Nicci French, will contain clues that will push readers to a real world location.
Week after week, story after story, readers will be invited to collect the hints and discover where the seventh story is hidden. The first week is based around a Google Earth interaction with a short story by Charles Cumming inspired to John Buchan The 39 steps novel.
Penguin's project is extremely rich and complex, and therefore, in my opinion, appealing, at least from a PR perspective. They have worked with alternate reality game designers Six to Start but, most of all, they've challenged with the project 6 young authors to whom they've asked to re-invent digital storytelling. It's a brilliant and clever way for a publisher to market literature!
The second example I'd like to bring is The Lost Ring, an ARG sponsored by McDonalds and the International Olympic Committee, for which there is even a trailer (see below).
The best way to discover this ARG is to read the game designer blog post that comments and explains the project and the work behind it.