Terrabrowser features built in support* for Garmin GPS receivers, and also supports Magellan receivers when combined with the freeware program MacGPS Babel). Download waypoints from your GPS to display on the maps, edit them, upload new waypoints to your receiver.
Terrabrowser aims to be compatible with the rest of the GPS world, so it uses the XML based GPX file standard which allows easy exchange (file, copy/paste) with other GPS programs for Mac, Linux, Windows, etc. which support this format. The NMEA standard protocol is also supported* which allows for a "live tracking" mode to display the user's realtime position on a satellite map (cached or not).* note that some of these features are not yet completed in the alpha version.