Sketchup is a simple to use yet effective 3D modelling tool recently acquired by Google. You can download a free light version or pay if you like to have the pro version. I downloaded the free version but haven't used it yet. But I was very impressed by the demo videos. Seems like fun :)
One popular use (and among the reasons of Google acquiring SketchUp) is to build 3D models of famous landmarks for Google Earth.