Craigslist is probably hosting their own tiles judging by the URL: http://map7.craigslist.org/t00/15/5242/12665.png

I think you might be right. I doubt they have rendered all the tiles though, it just takes up too much space to be worth it... http://wiki.openstreetmap.org/wiki/Tile_Disk_Usage

Looking at that same tile through my proxy (which is backed by the MapQuest map tiles), it is clearly different data.

Here is mapbox, which is also different... http://d.tiles.mapbox.com/v3/examples.map-vyofok3q/15/5242/1...

That's what I assumed. Any commercial provider would have made noise about providing maps, I would have thought- and been credited in the bottom right of the map. It's entirely possible to set up your own servers providing map tiles using Mapnik, I wonder if that is the route they took.