Two parts: OpenStreetMap, the website, aims mainly to be a database and a great editing interface. So the end result is mostly the data, which you can download in full ( http://planet.openstreetmap.org/ ). They have a few sponsors to handle the bandwidth required ( http://wiki.openstreetmap.org/wiki/Partners ), and a few meaty servers ( http://munin.openstreetmap.org/ ). There could always be more, and the base of partners could easily increase.
Part two is that _tiles_ and design are a different matter - OSM.org itself has low limits on the number of tiles you can use from their server before they start pushing you to use your own server or another service. MapBox is one of those services, and basically we handle a massive amount of work & bandwidth, and price out the service based on how much these things cost.
Yes. The problem is that Google has started charging now as well- at least, once you hit a certain threshold. They might have a deal available for non-profits, I'm not sure. It's certainly something I'd hope MapBox are working on, because non-profit mapping seems like a large (and worthwhile area).
No doubt, the bandwidth costs are expensive, though. I made my own custom map tiles (you can see a demo of them in use at http://www.taxono.my) and put them behind an Amazon CloudFront server, which makes it about as cheap as possible. But still, if my traffic gets huge, it could be an issue.
Have you tried Cloudflare?
It's great for anyone that don't want to combine it with data they can't release, but for anyone that needs to mash it up with proprietary data, it's a non-starter by design. I think a lot of people don't realize this, as I've seen more than one company use OSM data in ways that either puts them in conflict with the license or will force them to open data they've previously been very protective of...
But as others have noted, most people will need to host themselves or pay someone to host it for them once they reach certain thresholds.
(disclosure: I've got an interest in a company in the GIS space that can't use OSM data exactly because of the license, so I do have a vested interest in it)
On the other hand, if you produce new data by processing OSM data or mixing it with your own, then you are constrained by the bounds of Openstreetmap's license.
.. such as the moment you provide a function to generate PDF versions of the maps, or any number of other features that end up mixing the data. So in effect you either accept severely limiting your features, or you use other sources of map data.
If that's correct then that's fantastic.
Given that it's contradictory to what used to be claimed, it still makes me concerned about touching it without having a lawyer going over the license in great detail, though...
Wikipedia says no, but that may be an outlier. IMO they should start internalizing costs (especially for embedded maps) now before they get out of control, perhaps by throttling access so that high-volume users are encouraged to mirror and serve the data themselves. Coral CDN has a similar strategy: you can use it for free, but only a certain amount.
There are plenty of big users of OSM based tiles that would easily take down the OSM servers, so they have to either host their own or use some third-party to do so (MapQuest is surprisingly popular here, since they have capacity to spare and they'll serve OSM tiles up to some limit before starting to charge).
"OpenStreetMap data is free for everyone to use. Our tile servers are not."
Quote: "OSM does NOT pre-render every tile. Pre-rendering all tiles would use around 54000GB of storage. As the following table shows, the majority of tiles are never viewed. In fact just 1.79% are viewed."
Those figures are about a year old, though it does give a newer figure for space used which is about 1200TB or 2.7% of the total size estimated.
The one thing I don't understand is who pays for hosting it. Once everyone uses OSM, won't it be prohibitively expensive for a non-profit to serve it? Maybe I'm wrong and it's not that much of an issue. Does anyone have an idea how much it costs Google to serve the Google Maps API? Wikipedia is in the same situation, and they solve it with their donation pleas. Maybe it would make sense if a fraction of our ISP bill automatically goes to the most visited non-profit websites. They offer kind of a public good to everyone, after all.