To try it you need a web browser which supports HTML5, it means the latest version of one of the Firefox, Chrome, Safari or Opera.
On the Internet you can find plenty of free maps in the standardized GIS formats, which can be now transfered into your GPS device. Look at USGS topographic maps, USFS maps, or NOAA Nautical Charts, etc.
These geodata has already high quality georeference embedded, but they are using different coordinate systems and map projections then Google Earth and this complicates the conversion into the form supported by latest Garmin handhelds.
I am the author of MapTiler - a graphical application for Windows, Linux and Mac OS X, which allows extremely easy and user-friendly transformation of raster geodata into the web presentation (mashups) and tile overlay for Google Maps, Bing, Yahoo Maps, OpenStreetMap or Google Earth. You can download and use this software for free.