We are working on a set of simple base maps that we can use in different applications.
All basemaps follow the same URL structure (be careful as the URL might change in the future)
And if you need them over SSL use:
Most of these maps right now can not go further than zoom level 11, well, they can but dont show more detail.
You can explore the maps here with this Leaflet example (use the layer selector top right)
Here are a few base maps that can be used: