Haven't tried this personally, but I have heard some good things about it.
"Google SketchUp is an easy-to-learn 3D modeling program that enables
you to explore the world in 3D. With just a few simple tools, you can
create 3D models of houses, sheds, decks, home additions, woodworking
projects - even space ships. And once you've built your models, you can
place them in Google Earth, post them to the 3D Warehouse, or print
hard copies." 
