At Nathan Yau’s Flowing Data, I read that Stamen Terrain maps are now available globally, not just for the United States. Stamen uses Open Street Map and their tiles can be used with Leaflet.
One thing about relief maps is that they rub it in how embarrassingly flat the Netherlands are. For example, here’s the Oude Holleweg near Nijmegen (the nearby Van Randwijckweg was in this year’s Giro). It’s just a 70 meter height difference, but with gradients around 14% it’s one of the most challenging climbs in the Netherlands. Apparently, cyclists are not allowed to ride it downhill because that’s considered dangerous (update: I checked, it’s true, descending is not allowed). But if you look at the map, it doesn’t seem all that impressive.
Be that as it may, the Stamen Terrain maps «sure are pretty to look at», as Yau put it. Even when there’s hardly any elevation to show.