By far the best program I’ve come across is MyVirtualHome, an Aussie one. If you’re in Aus, then you can get this program free with the November 07 “Better Homes & Gardens” magazine … if you’re overseas, you can order the free disk from their website, – so long as you don’t mind paying for postage and handling. A work in progress (that means you’ll get more goodies as they add to the program), it has brilliant 3D renderings, which make most other programs look decidedly sad.
Google SketchUp is a free program you can use, you can do the house plans and then fill it with things from the 3D warehouse.
http://sketchup.google.com/