StreetComplete asks questions, with answers used to improve OpenStreetMap data
Really think this is a cool idea and have had it installed on my phone for more than a year, but somehow, I never really feel the compulsion to do anything more than open it up and think about how cool it is. Maybe would be nice to present a list of nearby simple quests to get new users started? A little intro/tutorial sequence may help bridge the gap and suck people in.