In active development

Name	Supported OS	Can be used offline	Map data source(s)	License	Notes
GMapCatcher	GNU/Linux, MacOS, Windows	Yes	CloudMade, OpenStreetMap, Yahoo Maps, Bing Maps, Nokia Maps	GNU GPLv2
Mapeo	GNU/Linux, MacOS, Windows (Android in development)	Yes	OpenStreetMap	GNU GPLv3
Maps.me	Android, iOS	Yes	OpenStreetMap	Apache 2.0
Marble	GNU/Linux, MacOS, Windows, Android	?	OpenStreetMap	GNU GPLv2	Built around the Marble library (license: GNU LGPL 2.1), to get directions while offline requires a plugin
modRana	Sailfish, Maemo5, Android 4.0+, MeeGo Harmattan, GNU/Linux (packaged for Fedora)	Yes, with OSM Scout Server	?	GNU GPLv3	forked from Rana
Navit	Android, iOS, Windows, MacOS, Linux	Yes	OpenStreetMap	GPLv2	some modules under LGPLv2
OpenMultiMaps	Android	?	GNU GPLv3
OpenTopoMap Viewer	Android	No	OpenTopoMap	Apache 2.0
OSMand+	Android, iOS	OpenStreetMap	Yes	GNU GPLv3	source repository contains non-free artwork licensed under CC-BY-NC-ND 4.0, removed from F-Droid version
OSM Scout Server	developed for use with Sailfish	yes	OpenStreetMap	GNU GPLv3	local map server allowing ordinary map apps to be used offline, known to work with: Pure Maps, WhoGo Maps, Poor Maps, modRana
PocketMaps	Android	Yes	Expat/ MIT
Pure Maps	Sailfish, GNU/Linux (desktop and mobile)	Yes (with OSM Scout Server)	OSM Scout Server, HSL, Sputnik, Photon (also Mapbox and others with API key)	GNU GPLv3	forked from WhoGo Maps
QMapShack	MacOS, Windows	Yes	?	GNU GPLv3 (or later, according to wiki)	active fork of QLandkarte

Discontinued

Name	Supported OS	Can be used offline	Map data source(s)	License	Notes
MapEver	Android	Yes	?	GNU GPLv3
Poor Maps	Sailfish	Yes, with OSM Scout Server	OpenStreetMap	GNU GPLv3
WhoGo Maps	Sailfish	Yes, with OSM Scout Server	OpenStreetMap	GNU GPLv3	forked from Poor Maps

Further resources: