Hot on the heels
of being acquired by Google, the
fine folks that make SketchUp have released a free version.
Now, at the moment it is Windows only, but a Mac version is coming soon (and I bet it'll be sooner than most Mac
products from Google).
The new version, called Google SketchUp (free), is slightly different than SketchUp Pro 5 (which isn't free). The free version is only for personal use, and you can't do things like print or export to a Quicktime movie, but you also can't beat the price.