Kites - what a fantastic idea!
I was just browsing Reddit (/r/Android) for recommended apps and I came across this fantastic app called Kites.
Essentially, it lets you leave messages around the map for your friends. They will get the message when they are at the specific location. So let's say that you're in a restaurant and you like a steak there. You can just leave a message to a friend and when your friend visits the specific location, he/she will get a message from you recommending the steak you loved. Or leave messages for your friends when they visit a city or other destination.
It is such a simple but elegant idea. You can sign in by using your phone number, G+ or Facebook. According to the trailer, it's also coming very soon on iOS.
PS. I love the fact that it uses a transparent navigation bar and notification bar on 4.4. Badland also does this. It's great that apps are already taking advantage of this.