Camera mapping is a really powerful technique that's used extensively in movies to turn still photos or matte paintings into very realistic 3D environments. A texture is projected from a camera onto the objects in front of it. It allows for some basic camera moves, like push-ins and pull-outs.

It's actually a feature in most 3D applications, and so far is not available in Blender. This tutorial explains an unorthodox way to get the same effect in the best free 3D application out there.