Orbiter Space Flight Simulator is the product of one under appreciated man, Dr. Martin Schweiger. He has now released Orbiter under an open source license (MIT).
You don't have to download all the textures, just get what you want and extract into root orbiter directory.
Download high resolution clouds (308.37 MB)You don't have to download all the textures, just get what you want and extract into root orbiter directory.
Download high resolution elevation data (11.78 GB)Get the Mars hi-res textures in one pack. Extract both the Surf.tree and Elev.tree files in: orbiter_root/Textures/Mars/Archive folder.
Download high resolution elevation data (2.63 GB)Get the Mercury hi-res textures in one pack. Extract files in orbiter_root directory:
Download high resolution mercury textures (4 GB)Get the Titan hi-res textures in one pack. Extract files in orbiter_root directory:
Download high resolution titan textures (166.77 MB)Get the Minor Bodies hi-res textures in one pack. Extract files in orbiter_root directory:
Download high resolution minor bodies textures (273.42 MB)