Google Sketchup - 3D modeling for FREE!
The SketchUp motto is "Dream. Design. Communicate." and is what they hope to accomplish with their software. I remember using this software a few years ago for some computer aided design (CAD) drawings that I needed for a presentation. Back then, the software was only available for a Mac. Now the software is dual-platform so you can sketch 3D on a Mac or PC.
While you can get a 'free' copy of SketchUp, which I'm sure comes with an installation of the Google Toolbar, the free version doesn't come with the full suite of tools that version 5.0 Pro does. The free version - currently in Beta - does allow you to model and explore worlds in 3D. With a few simple tools, it is possible to draw houses, sheds, decks, and other simple projects.
The software is very easy to use and has an amazingly quick learning curve. Drawing a 3D shape is as simple as clicking the desired shape and pushing or pulling it to create the 3D geometry. A quick tutorial on SketchUp can be found here. After you're done drawing, you can export it to Google Earth, post them to the 3D Warehouse, or even print a hard copy.
So the next time you need to do anything, try Google first because their bound to own a software company that does exactly what you need. All joking aside, the SketchUp software is very good and the free version has some sophisticated features you won't be able to find anywhere else.
If you find the need for software that is approved for commercial use, SketchUp Pro 5 is available retail for $495.