If you are in the middle of renovating the rest of your house, or even just updating the floorplan of a room, flooplanner.com can help you design and visualize your new space for free....as long as you aren't relying on the 3d walkthrough.
Admittedly, we have not yet used this planner, but playing with it for a few minutes showed it to be intuitive to use. In order to use the 3d feature, you do have to upgrade to the Plus or Pro version for $30.50 a year. At $79.95, Live Interior 3D looks like it might be a better option.
li>HGTV Floor Planner