Viking is a free/open source program to manage GPS data. You can import, plot and create tracks, routes and waypoints, show OSM, Bing Aerial and other maps, geotag images, see real-time GPS position (not in Windows), make maps using Mapnik (not in Windows), control items, etc. It is written in mostly in C (with some C++) & the GTK+2 toolkit.
Modular, touch screen friendly navigation system with GPS tracking, realtime routing engine and support for various vector map formats. Works on a wide range of devices, from computers to smartphones.
Multiplatform Ham Radio APRS and Mapping Program
NOTE: Code commits are FROZEN on SourceForge: New code is now at <http://github.com/Xastir/Xastir>. Real-time tracking & messaging of stations via radio/internet APRS data streams, w/125 map formats supported. Linux/FreeBSD/Solaris/MacOSX/Windows in any of 7 languages. Can also be used for mobile mapping w/GPS. MAILING LISTS ARE AT: PROJECT->WEB SITE.
Ertha is an open source GIS whose main focus is to guarantee easy extensibility and straightforward integration of custom modules. It provides basic geographic rendering and topologic analysis and exposes the main data structures loaded from shapefiles.