CartoDB[1] allows you to put your data on top of rich base layers. Use Mapbox or any other basemap provider. You can even create maps without a background.

Their Javascript library code is on Github[2], and they encourage people to collaborate. But it doesn't seem to have a license, I couldn't find one in the repo.

[1] http://cartodb.com/

[2] https://github.com/CartoDB/cartodb.js