Need help putting together a list of apps that use geofencing. Here's what I got so far.

Reminders - Apple's to-do list app. Can remind you to do something when you leave/arrive at a location.
Instapaper - downloads saved articles when you leave/arrive at a location.
Checkmark - more powerful version of Reminders, add timers on top of geofences, change size of geofences
Proximity - informs people when you leave a location using geofencing.
Twist - automatically tells people when you leave and arrive, estimates arrival times.
Prowl Owl - be informed when your Facebook friends cross or come near your geofence.
Bing - location-based reminders that mimic Apple's Reminders app.
Find My Friends - will get geofencing with iOS6.
News.me - downloads articles when you leave/arrive at a location.
OmniFocus for iPhone - location-based reminders.
Simple In/Out - an app that sets your status (ex., Are you in the office or out?) Geofencing can change status.


Apps that work with other geofencing hardware:

Garmin Tracker - buy Garmin tracking hardware and create geofences with this app. Receive notifications and text messages when that hardware (worn by a dog, in you child's backpack, etc) crosses the geofence.

Other:

Pay With Square - be alerted when a nearby store accepts Pay With Square payment service. Unclear if this is true geofencing.