Nope. They are doing exactly what I'm talking about which is proxying the tiles through their own hostnames so that they can get https support.

map#.craigslist.org. The leafletjs api makes this easy to do.

Do you know what underlying tiles Craiglist is proxying to? My impression is map{S}.craigslist.org is them rendering their own tiles from OSM data. But I'm not sure.

Correct.

Are you sure that OSM offer this service? Is it free? I can't find any details of it- it must be an utter bandwidth killer. Do you know where the details of the service are?

OSM absolutely do not provide free high traffic tile serving! http://wiki.openstreetmap.org/wiki/Tile_usage_policy

Perhaps a commercial OSM partner like MapQuest, MapBox or Cloudmade are providing the tiles?

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.

It seems as if it is free: http://help.openstreetmap.org/questions/3440/cost-of-commerc...

Though that answer is over a year and a half old.

That is talking about OSM data, which is quite different to OSM map tiles. The tiles are the result of the data, but the cost of providing data is miniscule compared to hosting tile servers.