FlightGear Flight Simulator is a free & open source flight simulator, it cover the whole planet with open data that covers the whole planet. Base of the airport come from the X-Plane airport database, as it's diffused in an open and free format. Planet earth datas come from various datas with open licenses. There are third party custom scenery that use satellite pictures too, like European Corinne one.
Since, at least one year, there is a process of integrating OSM data into the flight simulator main scenerary, at the same time, new open dataset of the terrain elevation, with freely available data are far more precise than before. Some improvement have also be made in scenery corner (like model animation on road or river streams. Everything will be available autogenerated with OSM data in next main scenery release (not sync and independant with next FlightGear release), but don't know exactly when, as only few people are actually working on developpement and testing.
There are already some custom scenery available using OSM data, and you can rebuilt it by yourself using tools freely available. Well you need some technical knowledge, some GB of RAM, HDD and lot of computing power during several days or weeks to render the whole planet scenery.
Some cities, are really well populated. Paris is probably the best populated with custom buildings, Shanghai, Tokyo or London have lot of custom buildings too, Hong kong a little les. San francisco or New York has only some custom building, most of building are modelized by standard parallepipded buildings with accurate highness. Lor of other big cities have one to few custom buildings. populated, with still lot of work needed, and some airport too. In the other side lots of airports are still missing (see it in China, where I edit a lot in OSM). There are several object in various place (electic pylons, castles, church, power plant, windturbine in place with standard objects too.
Some areas have regional textures, like Madagascar, Europe, mountains areas, etc...
For OSM features in FlightGear : http://wiki.flightgear.org/OSM
This is an answer to OSM & X-Plane.