This work was supported by the NSF Computation and Social Systems program and by the DARPA Control of Agent Based Systems and Automated Rule-Based Decision Making programs. I am grateful for many helpful comments from Chris Dellarocas (MIT Sloan School of Management), Feniosky Pena-Mora (MIT Department of Civil Engineering), and the members of the MIT Center for Coordination Science: Leon Grekin, John Quimby, George Herman, George Wyner, Abraham Bernstein, and Thomas Malone.