Garmin has taken the training wheels off their OS X application for the nüvi. Previously called Project Bobcat while it was in beta, it's now released as RoadTrip. The 63MB download is available on their Mac software page and has the following features:
RoadTrip allows you to transfer waypoints, tracks, and routes between your Mac and Garmin device and manage your data using your Garmin maps.
RoadTrip provides the ability to search for points of interest from the convenience of your Mac and then send the locations to your Garmin GPS.
MapManager copies maps and unlock codes into the right place to be accessible by RoadTrip and MapInstall. To migrate the maps from your Windows PC, download MapConverter onto your PC and follow the instructions.
Download and install MapConverter for Windows to convert your unlocked PC maps for use on your Mac.