For my starting geocaching adventure I bought a Garmin Etrex 20 GPS. The only map it has on board is a demo and only shows major roads. So I needed a topografic map with all the streets and cart-tracks for easier navigation to the several caches.
After some Googling, I found the solution for free: OpenStreetMap for Garmin devices.
You can download there the maps for BaseCamp or generic maps you can install directly on to your device
Update 25/11/18:
I recently upgraded to an etrex 25t, but the pre installed maps gave an error while trying to update them, so I looked for a temporary solution for having up to date maps until I get this problem sorted with Garmin customers service
Free topographic maps:
http://www.frikart.no/index.html
Free biking maps:
Free biking and hiking maps: