I noticed that the ID editor allows the user to enter a URL for for a tileserver to use as a base map. I am spending time lately following up on craigslist user Notes submissions and sometimes it is helpful to see the area as the user did when they wrote the note.

So I added the CL tile server as a custom layer. The URL for this is:

http://map.craigslist.org/t03/{z}/{x}/{y}.png

The t03 is the current version of the tileset which changes when I add new data and generate new tiles. This value will change but we only use t00-t09 so you can try one of those or come to craigslist.org and take a look at a map (currently only US and Canada) to see which tileset we are on.