Viking is a free/open source program to manage GPS data. You can import and plot tracks and waypoints, show OpenStreetMap and/or Terraserver maps under it, download geocaches for an area on the map, make new tracks and waypoints, see real-time GPS position, etc.
It is written in C with the GTK 2 toolkit, available for Linux, other POSIX operating systems, and Windows, and is licensed under the GNU GPLv2 .