Very nice and completely routable maps based on OSM data by Lambertus
This map is based on data from the OpenStreetMap Project. Lambertus offers probably the easiest and most convenient way to download OSM data for your Garmin GPS. You can download individual tiles or select a number of tiles you want to download through a nice map/online viewer. The maps are routable and also come with a Mapsource AND a RoadTrip installer. However as the capacities are limited there sometimes (depending on the time of day) is quite a wait until your request is processed and your individual mapset is build.
OpenStreetMap works that way, that everybody can upload their GPS tracks from which maps from all over the world are being created. This means that the quality of the map depends on the tracks uploaded by the users (you're welcome to contribute to the project by uploading your tracks as well) and therefore the quality differs greatly from country to country, region to region and city to city. As there are continuously new tracks being contributed the maps are getting better and better every day, so if a map of a region is not as detailed as you had expected either join the community and help by contributing and/or come back in a few weeks or months and you will most likely find a better map with more data and information than there was before.