A well-developed and stable application that is offered under GPL (so free). For GPS tracking, it is excellent for removing "stray" points or tracks where your GPS unit was just getting a lock, but not a very good one. It also does a fantastic job of correlating photographs to your GPS track and encoding this data into the EXIF file to store permanently with your photos. Add to that the ability to put OpenStreetMap data shown beneath your tracks, and to export too and it is an almost unbelievably good package.
The later version (7) is now available and gives better charting, distances and speeds appear more precise and you can use different slippy maps from OpenStreetMap (try OpenPisteMap to see how you were skiing on the pistes, for example).