Benoît Mandelbrot whose classic work on fractals as well as more recent work questioning the Efficient Market Hypothesis offers a lasting contribution to positive economic theory. While the committee is likely considering Eugene Fama and/or Kenneth French (of Fama-French fame), we believe they should instead consider Mandelbrot (or at a minimum split the award between Fama, French & Mandelbrot).
Robert Axelrod whose work on the evolution of cooperation is among the most cited work in all of the social sciences. Iterated Prisoners Dilemma as well as concepts such Tit for Tat are part of the cannon of almost all introductory courses in game theory.
Robert Shiller for his contributions to behavioral finance including his work challenging the Efficient Market Hypothesis. Of course, Shiller is also well known for his work on the real estate market with Karl Case (including the Case-Shiller Index). This also represents important work worthy of recognition.
Elinor Ostrom for her work on public choice theory, common pool resources and collective action. Her work has offered a substantial contribution to political economy as well as institutional and environmental economics. {Note: (Ladbrokes places her at 50 to 1)}.
UPDATE: ELINOR OSTROM AND OLIVER WILLIAMSON WIN THE 2009 NOBEL PRIZE {In Our Estimation, this is a Very Appropriate Decision }