Floor Planner is a free tool for building floorplans.  Not only is this a fantastic use of flash in an online app, but it will soon include 3d (above).

http://www.floorplanner.com/3ddemo

In addition, users can also share their ‘creations’ with others. And by including a user-supported forum, the developers have made it easy to share and contribute to the applications development.
 
http://www.floorplanner.com/weblog/
 
Jim Bachalo