Google SketchUp is a bit finicky, but overall it serves as a great tool to build rough spaces quickly to use as underlays. It also has a decent "color blocking" feature that allows you to color walls and surfaces with paint colors or textures (brick, roof tiles, fencing, etc.). This has allowed us to visualize how different rooms would look with various color schemes.
If you're so inclined, download SketchUp for free and play around with it a bit. I found it really tricky to use until I switched all of the keyboard shortcuts to mirror those of the Adobe products (Photoshop, Illustrator, etc.). SketchUp certainly has its limitations, but for a free program - it's pretty darn useful.
POV from the front of the Living Room, looking through to the dining room. Don't you just love the sofa and painting above the fireplace?