OsmAnd Maps & Navigation is a free map and navigation application that utilizes OpenStreetMap data.
Pro Excellent general map viewing
Sometimes one needs more than just navigation, they have a need to view a map or specify a specific route. OsmAnd provides a very nice, detailed map. This can be good for seeing what is around you or showing a place you are talking about to a friend.
It also provides a means to add waypoints on the map which can be used to dictate a specific route (not simple enough to do routinely).
Pro Based around crowdsourced OpenStreetMap data
OSM provides a map editable by everyone; just like the web-based encyclopedia, Wikipedia.
Using a community edited map allows one to correct isses they encounter. Combined with the monthly turn around, these edits are added for navigation quickly.