I don't know if anyone else has tried Sketchup, but I'm finding it to be quite an asset. It's not really a rendering tool, but you can make 3D models pretty easily. It can make cylinders and geodesic domes, but it seems to be able to deal with boxes a lot easier.
To me, it's very good for revisualization, and using print-outs for construction lines. You can even simulate different camera lenses. I don't think I've ever encountered a free download that's become an actual tool before. But after a period of awkwardness, I've found it fairly simple. There's a "For Dummies" book that just came out, and I'm going to have to buy it!