Reprojected Raster Tiles
Zoomable raster world map on an Albers equal-area projection.
Any Tiles, Any Projection
This approach will work for any multi-resolution Mercator-based tileset that is subdivided recursively in the form of a quadtree, combined with any inverse projection, i.e. a mapping from 2D to spherical coordinates.
This means that you can take beautiful raster tiles, say from Stamen, and reproject using a projection of choice: