Help to improve the OpenStreetMap with StreetComplete!
This app finds incomplete and extendable data in your vicinity and displays it on a map as markers. Each of those is solvable by answering a simple question to complete the info on site.
The info you enter is then directly added to the OpenStreetMap in your name, without the need to use another editor.
Cool I loved it when Google Maps allowed contributing stuff. There are so many little annoyances where you get incorrect directions that could be fixed so easily by crowd sourcing, but are too small to fix otherwise.