Google acquired @Last Software, the creator of SketchUp, a powerful tool for creating, viewing, and modifying 3D models quickly and easily. SketchUp was developed to combine the elegance and spontaneity of pencil sketching with the speed and flexibility of today's digital media.
Although Sketchup is a commercial software (it costs $495), you can try it for 8 hours (31.2 MB, available for Windows and MacOS X).
One reason for which Google acquired this company is that SketchUp has a very nice plug-in for Google Earth. You can create virtual models for buildings, parks or streets.