Apple, Google and others have been offering geofencing-related services for a while, and now Esri solidifies their offer with the public beta version of Esri's Geotrigger Service.
From the official entry: "How does the Geotrigger Service work? An invisible area drawn on a map is set to have an action or message associated with it. When your mobile device crosses into the “trigger zone” the Geotrigger Service sends a location-based message to that device, or even notifies your server for custom events. […] The Geotrigger Service runs in the cloud. […] Free while in beta."