[DjangoReinhardt]

I think Ingress is a close match to your description. Also, Lyft and other ride-sharing apps work along the same principles, I think. Similar techniques are also being used for the new-age anonymous messengers, but I may be wrong about those.

I knew of people attempting to create 'discount floats' based on geo-location - as in, a retailer with the app would 'float' a specific discount at a specific point and anyone with the app who happened to pass within a certain radius would get a notification/coupon of the discount.

However, it didn't pick up here in India because smartphones and app downloads weren't all that ubiquitous two years ago, when they tried to implement this idea. Now, the results may be different, who knows.

[EC1]

Wonderful. I'm founder @ parcel.io and we're doing just this. For regular people to just discover data. We have a few built in layers, one for history, politics, etc. When you enable them, you can view a layer of locations in that categories specific color. Lets say I walk beside a certain monument, I can grab data of it (pictures/videos/text) and learn more about it by being in proximity.

[sharemywin]

is it similar to foursquare?

[EC1]

Nope. You don't check in everywhere. It's literally like picking up/dropping off/tracking packages, just it's virtual data instead of a physical package.