Gpsbabel

To communicate via USB use usb: as the filename on all OSes. Thus, to read the waypoints from a Garmin USB receiver and write them to a GPX file:

gpsbabel -i garmin -f usb: -o gpx -F blah.gpx

If you have multiple units attached via USB, you may provide a unit number, with zero being the implied default. So if you have three USB models on your system, they can be addressed as usb:0, usb:1, and usb:2. To get a list of recognized devices, specify a negative number such as:

gpsbabel -i garmin -f usb:-1

When reporting problems with the Garmin format, be sure to include the full unit model, firmware version, and be prepared to offer debugging dumps by adding -D9 to the command line, like:

gpsbabel -D9 -i garmin -f usb: -o gpx -F blah.gpx