Myself and the other folks at Stamen Design are getting ready to officially publish a new, global version of the free to use, OSM-based, terrain maps that we launched for the U.S. back in 2012. The Knight Foundation funded the initial work and we’re grateful to them for continuing to support our ability to generate worldwide maps for free, to all. If you'd like to take a look before we launch publicly, we'd love beta testers. In particular, we’re looking for people to drive around the world and help seed the generation of all these new tiles. We’re also looking for people to identify problem areas that we may not have seen yet - it’s a big project and we haven’t been able to look at even a small portion of the globe yet.
Send us an email at firstname.lastname@example.org, and we'll send you a link to the beta very soon.