Google has released yet another beta software package: SketchUp. SketchUp is an application that allows you to create your own 3D objects, color and texture your designs, use real-time shadowing effects and grab from a large library of pre-configured objects to add to your design. And as is always the case with most of Google's software, it is completely free. There is also a commercial version called SketchUp Pro 5 which has a host of additional features, but you may want to take a stab at the free version first to see if it's suitable for your needs. From Google:

Google SketchUp (free) is an easy-to-learn 3D modeling program whose few simple tools enable you to create 3D models of houses, sheds, decks, home additions, woodworking projects – even space ships. You can add details, textures and glass to your models, design with dimensional accuracy, and place your finished models in Google Earth, share them with others by posting them to the 3D Warehouse, or print hard copies. Google SketchUp (free) is a great way to discover if 3D modeling is right for you.