GPSS seems to be the leader in this space, because it provides maps for over 150 countries and voice guidance in twelve languages. GPSS supports pretty much any GPS with an NMEA interface.
Easy GPS is a good package if your requirements focus on waypoints, routes, and tracks. It's by the same people who make ExpertGPS and GeoBuddy. EasyGPS supports Garmin, Magellan, and Lowrance handheld GPS units.
EasyGPS lists your waypoints in the left window, and displays a plot of your GPS data on the right. EasyGPS can be used to back up and organize your GPS data, print maps, or load new waypoints onto your handheld GPS.
Other free GPS software includes:
Waypoint+, but it has not been updated in several years.
GarWaypt, but it is less mature than EasyGPS.
Linear Mode
