OpenStreetMap is a database full of really useful free mapping data. There are several views of the data, such as the Standard map and OpenCycleMap you may have seen on the web or via a phone app.
Some clever people have also converted the data for use on Garmin GPS devices. There are an overwhelming number of maps which all look different. The best map I have found for cyclists is OpenMtbMap. Screenshots are hard to come by so here are some running on a Dakota 20 ...
The overview map (see on OSM). Note the scale is 8km. The Dakota 20 struggles to redraw the map at this scale, taking around 5s to draw the map. It gets quicker if you zoom in.
Planning a route is too painful on such a small screen so I do that with a paper map or at home, saving a gpx track of a planned route and copying to the Garmin.
OSM), the map data starts to come alive. The green, orange and yellow routes are standard roads, primary(A), secondary(B) and tertiary(C).
Solid black is an unclassified and paved road. Eg north from Rampton.
Other OSM-on-Garmin maps show some unpaved routes but not with any great detail, it is really hard to tell the difference between a smooth track and a muddy footpath. OpenMtbMap does a better job. More in the next pictures.
Red is a track of some variety. In the UK this does not mean a legal right of way. All of the tracks on this screen can be legally used. The marking, solid or dashed is telling you something about the quality of the surface. Haven Drove G5 is a Grade5 track in OSM data on a scale of 1(smooth) to 5(bumpy and having a soft surface).
Brown and Grey (in the lower left quarter) is a bridleway. In the UK legal for bicycles to use but not motor vehicles and is an important distinction.
Dotted black is a footpath. In the lower right quarter there are a pair running either side of a drainage ditch (blue).
OpenCycleMap). The thick black marking denotes a cycle route.
Note when selected with the pin, the name appears at the top. The menu reveals the full name "Ucl Ncn Natonal Route 51".
OSM and the OpenCycleMap layer. There is a lot of detail. Working left to right:
Green dashed is mtb:scale=1. This is what I have marked as good singletrack for mountain biking. It is useful to distinguish from other paths that are either uninteresting or for walkers (appearing as solid dark red but not on the screenshot).
Thick orange is a B road.
The trail marked with a blue edge is a marked cycle route known as the Lime Burner Trail, with this small section named The Beast.
The three brown trails are all highway=path in OSM speak with subtle variation of surface.
The red dashed and straight sections are tracks. The whole forest is on a grid system and these are everywhere. The Fire Roads are wider and harder packed with many marked solid red in OpenMtbMap.
OSM). Also note the brown dashed footway which is a useful pedestrian route between houses in my village.
Sadly, additional information such as phone numbers does not make it into the Garmin.
A few nuggets of information:
It is worth noting that postcodes are not generally in OSM data so you cannot navigate like you would with a car satnav. Additionally, address searching by road name also does not work in my Garmin. The reason is because to get OSM into a Garmin format, the data format has been reverse engineered. I believe you can get this working if you upload the map via Garmin's MapSource software. The work around is to find the road you want on the map, highlight it and navigate to it. Also, auto routing is very hard to do when you go off road, mostly because of the data. OpenStreetMap data is going to give you the best chance if you want to do that but you should be aware of the quirks of OpenMtbMap's routing. One example is that it will not route you on Primary Roads (unless specially tagged). This is a good thing in my opinion but a quirk is that it prevents my Garmin routing my over this staggered junction. If you are a road or touring bike rider you probably don't want to install OpenMtbMap. The same author has created VeloMap which should not route you over aggressive mountain biking routes.
If you want a plain map for just road use, the Talky Toaster OpenStreetMaps are very good. I switch to these if I need to use the Garmin Dakota on my motorcycle. You can see offroad trails in the screenshots but you'll see they are all simple grey lines.
Installing OpenMtbMap on your Garmin: The way I install this is to download the UK build from OpenMtbMap, run an exe to build me an IMG file, plug in the Garmin via USB, and drag and drop the file onto the Garmin disk.
As a final note. Around Cambridge we are blessed with very good quality OSM data. When I travel to other locations such as the Peak District I find lower quality map data. This is most often bridleway routes marked as footpaths, and occasionally missing routes entirely. My pattern is to plan the route (on BikeRouteToaster) using an OSM map view, falling back to Google Earth if unmapped, then save a gpx file of the planned route. I follow that on the ride then upload the missing gpx data to OSM (you should not copy data from other map sources, this keeps OSM data free).