Neuroprosthesis News reports an interesting Ken Parlin article on single and multi-wheeled balancing robots entitled Rosie the Robot - A Question of Balance. The article includes Java applets that simulate a variety of balancing principals in two of Ken's projects:the Robotics Operationally Self-Balancing Intelligent Endtable and the One Wheeled Balancing Robot. If you're looking for a more practical project, David P. Anderson of the DPRG has built a two wheeled balancing robot called nBot.