This app won't be easy to describe, but here goes. Cutetown is a free iPad app equipped with an aerial photo of San Francisco and parts of the Stanford University Campus. You can select buildings from the 2D photo, then extrude them in height, width and depth before adding pre-formed 3D roofs, windows, doors, etc. When you are done, a 3D building has been added to the map and you can go back and grab another. It's not a serious map-making or 3D app, but it's kind of like having some control over a Sim City layout without the animation.
The app features a tutorial -- which isn't always clear -- after which you're sent on your way to start creating buildings. You have to sign up for an account, apparently so users can collaborate with others to crowdsource the finished map. I don't like the idea of signing up for things without an explanation of how the information will be used, so a few points get shaved for that.