SketchUp- for free
Google is giving away a "personal" version of SketchUp, free. Limitations are similar to GoogleEarth (no extra format export).
SketchUp is a simple but powerful tool for quickly and easily creating, viewing and modifying your 3D ideas.
+ Click on a shape and push or pull it to create your desired 3D geometry.
+ Experiment with color and texture directly on your model.
+ Real-time shadow casting lets you see exactly where the sun falls as you model.
+ Select from thousands of pre-drawn components to save time drawing.
Watch the fun tutorials to learn how to start using this tool.
Available only for Windows. Although it does say "Macintosh: Coming Soon" :(