This process just got a whole lot easier. I used to have to copy and paste latitude and longitude as waypoints from an on-line map into my Garmin Map Source application, export the waypoints to GPX format, use GPS Babel to convert the waypoints to points-of-interest (POI), and finally use Garmin's POI Loader to load the points onto the GPS.
The Garmin Communicator Plugin makes this task a lot easier. You just need to highlight a marker on a Google Map, right click, and select the Send option. If your GPS is plugged into your PC, it'll be discovered and allow you do download the location directly. The location is installed as a waypoint, rather than a POI, which is a little bit of a disadvantage because waypoints are limited to a shorter text strings than POIs on many GPSs.
I've found the Communicator Plugin works best when you've only got a single marker on the map. If you have multiple markers on the maps, as when you do a Google Map search, you don't get to edit the name of the waypoint. To create a single marker you can edit and download, it's best to either use Google's My Maps or enter the coordinates directly into the search bar.
I generally load my GPS with the locations of interesting stores and restaurants, sights, our hotels, and ATM locations near the hotels. Of course with any piece of electronic gear, you need a back-up, so I always carry paper maps in my knapsack.