UPDATE: MapTiler 0.6 supports Garmin!Download the latest version from http://www.maptiler.com/
See the video tutorial:
Step by step documentation: http://www.maptiler.com/how-to/garmin/
Garmin rolled out beta version of firmware for their latest generation handheld GPS receivers allowing you to display scanned paper maps and another raster data in their navigation devices. They have chosen to do that with a simplified variant of Google Earth's KMZ format, so you can prepare maps for your Garmin even in Google Earth.
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.
The application now supports direct export into the Garmin Custom Maps KMZ. If you have one of the supported Garmin devices you can try an example map: the USGS map of Grand Canyon encoded for Garmin GPS KMZ (9 MB) or the alternative from USDA Forest Service. (10 MB)