http://sketchup.google.com/

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. ©2007 Google

Google SketchUp is also very useful in education. Check this out.