Geocaching.com, the biggest and most 'official; geocaching website has just announced that they will switch to OSM maps.
They state that they have been contacted by Google and would have to pay for their map usage in the future. Probably they didn't like that idea very much.
As a geocacher myself, I'm quite happy about the move. Their premium account already costs some money, and I wouldn't want that price to increase because of google.
Until now, OSM maps were an option on their main map display. lately they switched their static map thumbnails to OSM, now they moved the main map too.
They say they are using leaflet, user report that for know the tile source is MapQuest. But future plans seem to point to an own server, as they already talk about the new features they can implement with OSM, that weren't possible before.
Just do yourself a favor and don't dive into the comment section of that original blog post. A particular 'unhappy' customer is flaming heavily against OSM.