Check out this free software for PCs - Mac version coming soon...
Google SketchUp (http://sketchup.google.com/product_suf.html) 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 3-D warehouse, or print hard copies.