This is pretty amazing:
Google SketchUp 6 is a 3D modeling software tool that’s easy to learn, simple to use, and lets you place your models in Google Earth. Are you remodeling a kitchen, landscaping your back yard or adding a deck to your home? Google SketchUp makes it faster, easier and a lot more fun. From simple to complex, from conceptual to realistic, Google SketchUp helps you see your vision before you build it.
To get an idea of how powerful and simple to use it is, someone has a bunch of demonstration videos up.
(Via Mark Frauenfelder at Boing Boing.)