One of my preferred 3D tools is Terragen. Thanks to this free (for non-commercial use) terrain generator, I could create all the open terrains that you can see in my demos, but also all the backgrounds.
It's very easy to create a beautiful landscape with mountains, clouds and a shiny sun. After tuning some parameters, it's only necessary to take six shots of the scene with an aperture of 90 degrees in the six main directions, then you can load the images in APOCALYX and apply them to a sky-box. Only two tricks are necessary to merge the background to you actual scene: choose the correct sun inclination, so the lens-flare will appear in the right place, and choose a fog color that corresponds to the color at the hills foots, so objects disappear slowly with distance.
Of course there are also ready-made backgrounds out there. For example, you can see above and below two cube-maps created by Humus from real photos.