Stamen recently released a set of free open source web mapping services that are both fast and beautiful.
Watercolor (my favorite) is a great map service for anyone looking to add some artistic flare to their maps. The basemap incorporates hand-painted brush textures and high-level cartographic algorithms to create a map that honestly looks like someone sat down and pained every inch of the planet in watercolor. The map is absolutely stunning.
Terrain is the perfect open-source alternative to Google’s terrain map. It looks great at all zoom levels and visually has the birds-eye effect fully locked in.
Toner is a wonderful basemap for anyone providing a print page for their maps. It’s designed using only black and white with focus on labeling and textures. You can really make a dataset pop against it if you add a splash of color, like selecting a building in red – for example.