iOS: Location-based communication (like using Foursquare to find Wi-Fi passwords) is nothing new. Knit, however, lets you target messages to specific people when they arrive at a destination.
The app allows you to tag a message while you're at a specific location and choose who can discover it. When they arrive at the place you left the message, it will appear on their phones (in the Knit app, of course). While it seems to work best for leaving messages to other members of a household, there seem to be plenty of fun uses for it.
Knit [iTunes App Store]