OpenTopoMap
OpenTopoMap is a topographic map out of data from OpenStreetMap and SRTM. If you are interested in building your own OpenTopoMap, see the beginner's guides for a tile server and for Garmin files.
Mapnik
The main OpenTopoMap is a online rendered raster map. It can be used with the web interface on https://opentopomap.org, which gives you nice features like a search function or loading your gpx tracks. Futhermore, OpenTopoMap can be included into other applications. See https://opentopomap.org/about#verwendung for information on usage. The license of the online map is CC-BY-SA.
The online renderer is based on Mapnik. All necessary files are available to build your own OpenTopoMap server.
Garmin
Since summer 2014 there is a Garmin edition of the OpenTopoMap. They can be downloaded from http://garmin.opentopomap.org. The license of the Garmin maps is CC-BY-NC-SA and therefore reselling is not allowed. Here are some screenshots:
