VeloMap.org -Roadbike – Bicycle – Maps based on Openstreetmap
free cycling orientated maps for most countries.
Waiting on mine to download, then I’ll report how good they are.
ok, well the Aus maps a few others require a donation of 20 euro, but most of the other maps are free, have contour lines, good quality and using BaseCamp (free) from Garmin you can upload as many individual maps onto your unit as you have space.
Not quite open source but I think you’ll find them useful… 
And you might find more useful stuff here…
The Pirate Bay - The galaxy’s most resilient bittorrent site
Hmm interweb censorship… Pm for better options or type garmin in to the search bar of the bay where bearded, rum swilling, skull and cross bones kinda guys frequent…
just found this one too
Free worldwide Garmin maps from OpenStreetMap
No topo lines but you can select bike specific at the top, then highlight the map tiles you want from anywhere in the world then request that map, its assembled and you get an email when its ready, download then just copy over to the SD card on your garmin, no programs needed, no fuss.