Vicinity Alarms

I had this idea like 8-10 years ago. Clearly if I wait long enough, someone will build it for me:

Android Developer Challenge Gallery - Android:

SynchroSpot’s mission is to provide users with a whole new class of location-based mobile software. Location-based reminders allow users to input reminders into their phone and the phone will asynchronously alert them whenever they are near the given location with their reminder (e.g. Pick up milk the next time you’re at the supermarket).