We've had to block some uses of the OpenStreetMap Foundation tile servers.  This article describes what is happening and why.  This article also describes how you can adapt if you are effected.

We started creating rendered images of our map data as a way to encourage our data contributors.  Mappers enjoy seeing the results of their surveys on the OpenStreetMap web site, and they can be inspired to map in their neighbourhood the things they see other mappers surveying in other places.  Mappers loved the tile server when it first appeared.  Potential users often looked at the map in 2006 and said, "Hey, why is there a huge blank spot where my town should be?"  Some of those potential users became the long-time contributors that we all know and love.

As more contributors mapped more neighbourhoods, more blank spots started to fill in.  More potential users became actual users, and OpenStreetMap tiles started to appear in more places.  The tile server became even more popular when rapid updates were enabled.  Rather than updating the map every week, parts of the map could update as contributors added data.  If you remember the weekly updates you also remember that funny tingle you had the first time you mapped something and it appeared on the map immediately; it seemed like magic, didn't it?

The OpenStreetMap Foundation has had a tile usage policy for some time.  From September 2008 it has been explicitly stated that bulk downloading of tiles was discouraged.  OpenStreetMap kept growing.  More people came to understand the awesomeness of OSM tiles.

Also in 2008, the Ordnance Survey started serving map tiles to users.  In July of 2011, Ordnance Survey served their one billionth tile to a user.

OpenStreetMap serves a Billion tiles every eleven days.

So we know a thing or two about providing awesome maps to users.  We do it all with the crowd-sourced  data from our contributors around the world, the volunteered time of our sysadmins who keep our servers running, and the generous donations of servers and bandwidth and funds.

We've had to become more restrictive of the use of our tile server over time.  We've limited how many tiles you can consume in a period of time.  These restrictions only effected the most-demanding of tile consumers.  The everyday mapper never ran into a problem getting tiles to add data to OSM.  That allowed the growing number of users to continue to have access to OpenStreetMap tiles without our resources being monopolized by one or two bad actors.

More and more mobile applications started using OpenStreetMap tiles.  Many of them included a bulk downloading method so that tiles could be saved on the device at home, rather than downloading tiles at a punitive data rate.  That bulk-downloading has always been problematic for OpenStreetMap because a single user will consume hundreds of times the resources of an average user.  there are so many applications using OSM tiles, with so many users making unreasonable demands on our resources that it is affecting the quality of service for the average user.  And that's not fair.

So we've started blocking the applications that are causing us the most trouble, in addition to blocking users with problematic specific behaviours.  We regret it, in a way.  After all, we map because we want people to be able to use our data.  But it has to be used in a way that everybody can share.  We can't have a small number of people consuming all of our resources.

So that's why some people have started to see the "prohibited" tiles on their maps.  Overuse.  Or mobile applications that are causing overuse by a group.

So what can you do about this?  How can you get the wonderful OpenStreetMap tiles for your mobile device?

You can run your own tile server.
You can use an application from a developer that provides a tile server for you.
You can use tiles created from OSM data, provided by a server that permits bulk downloading.
You can buy tiles from a commercial provider.