I've got a Garmin Nuvi 260W that can connect via USB to my Windows computer. I've connected it and the device was detected and drivers autoinstalled by Windows.
My question is, ... what now? What software can use this GPS device?
On the garmin website you can download software for your device
Use Windows (file) Explorer to browse the file directories. You can discover where photos and MP3 files can be stored.
The Garmin Software is free, however the Maps are not. The catch to the Nuvi is that the maps can't be read of the GPS, and needs to be separately purchased and installed.
You can however pull your travel logs of the GPS and I am in the process of writing a little application to parse these and then map them on with Google or Live Maps.
There is a free Mac RoutePlanner available on the Garmin site as well which I haven't had a chance to look at yet.