For GIS-types, of the more interesting Android apps to emerge from Google has been the geo-game Ingress. It is a rather clever augmented-reality app that leverages Google's extensive map and POI datasets to overlay a freedom-fighter plot line on the real world. Players choose to be on one of two teams, The Enlightened or The Resistance. The game creates "portals" at various POI location (lots of church parking lots) and players travel around "hacking" - or claiming them for their team.
This type of game experience seems like the perfect application for of Esri's Geotrigger API... if they've got those power management issues worked out.
Today the app arrives for iOS as well... check it out here: Ingress