I am a contributor on OSM, and Organic Maps is the only navigation app I use. I primarily contribute in the UK, but I have also contributed in some other locations (see 'Where Did You Contribute?'). (OnwardTravel on OSM and OHM).
I am generally working on adding buildings (houses, garages, sheds), fences, driveways, pavements, trees, fields, grass, and gardens, as well as addresses (i.e. mostly nitty gritty details).
View historic maps of mostly just Great Britain that are out of copyright. The Ordnance survey 6 inch, 25 inch and 1:250-1:10560 are the most useful and interesting (in my opinion).