Map Tiles from Stamen (http://maps.stamen.com/) are published under a Creative Commons Attribution (CC BY 3.0) license. The How-to guide at Stamen shows map URL templates for three different types:
At the moment of writing toner and watercolor tile sets are available worldwide whereas terrain is only available for USA.
How to use it in uDigTo add a new Layer click on Menu Layer -> Add .. to open Add Data Wizard. Choose Web Map Tiles in the list followed by button Next >.
On the next page uncheck Mapnik first and check Custom Server in the left panel.
Copy one of the map scheme URL's listed above and paste it into URL field on the right. To finally add the layer to the current map push Next > Button again. The layer is named Custom Server. To rename the layer, just right click on the layer in the layers view and choose Rename.