GitHub repository: rnorris/viking
Primary language: C
Description: Viking is a free/open source program to manage GPS data (including GPX and KML files). You can import and plot tracks, routes and waypoints, show OpenStreetMaps (OSM), Bing Aerial and other maps, generate Mapnik maps, geotag images, make new tracks, routes and waypoints, see real-time GPS position, etc.  It is written mostly in C with the GTK+ 2 toolkit and some C++.