There's a nifty urban ARG called London Action coming up next week, and it sounds quite appealing:

"You’ll be given a device equipped with GPS and image-recognition software free of charge. Using this tool you’ll pick up streaming videos and virtual clues hidden in the nooks and crannies of the urban landscape. Every lead you find takes you deeper into the history of the resistance and presents a new challenge."

Interestingly enough, it's not something backed by any big companies - instead, it's all about charity. Picked it up from Londonist. If it all still seems a bit confusing, you can find guidance here.