I've been reworking our kitchen floor plan using QCAD, which is a good piece of CAD software, but somewhat tedious for floor plans (you have to create both sides of each wall, for example). I suggested bulwynkl look for special floorplanning software for his extension planning, then I suggested it to myself. After some digging I found Sweet Home 3d, a java based tool which has been remarkably helpful. It lacks good snapping rules, but otherwise I found putting together floor plans fun and simple.
As you can see from this photo, it is 3d, and comes with a decent set of basic furniture. It also does global illumination allowing you to identify dark spots in your plans.