Google Sketchup let's you get as into a design as you want. You can do a basic layout or you can go whole hog and design an entire room with paint colors and everything.
Arrange a Room from Better Homes and Gardens is our go-to for a quick look at a new layout. It's simple but you can't do much more than see shapes and a room.
My Virtual Home
Design my Room from Point Click Home
Image from Jack's Think Big