Gowalla is a mobile social network where you can check in from various places and "pick up" and "drop" virtual goods.
This app is very well designed. I like the fact that you can add pictures to the venues you visit, which is a nice touch. However there doesn't seem to be a way for venues to leverage it as far as I can see today.
There are virtual items that are discovered during check ins, but there isn't a clear purpose as to what to do with these items once collected.
It's cool to know where my friends are and read the notes about the location they are in, I just don't see the real world benefit to venues just yet. It shows lots of promise and user adoption.