Stanford Encyclopedia of Philosophy entry  
   This is a very complete description of the prisoner's dilemma and its variations.
   http://plato.stanford.edu/entries/prisoner-dilemma/

Prisoner's Dilemma by the constitution web site  
   Some links on ethical and political aspect of the Prisoner's Dilemma.
   http://www.constitution.org/prisdilm.htm

The Taumoda Lab  
   This site offers a way to learn how to study the evolution of cooperation through 3 models, based on the Prisoner's Dilemma. (Java applets included).
   http://www.taumoda.com/web/PD/lab.html

Xerox PARC  
   This is a directory containing a lot of papers on Dynamics of Computation Area (Evolution of cooperation) from the XEROX PARC.
   ftp://parcftp.xerox.com/pub/dynamics/

Annotated Bibliography on The Evolution of Cooperation  
   A large annotated bibliography on the Prisoner's Dilemma and cooperation, compiled by Robert Axelrod and Lisa d'Ambrosio.
   http://pscs.physics.lsa.umich.edu/RESEARCH/Evol_of_Coop_Bibliography.html

The Prisoner's Dilemma  
   A simple yet powerful model for the problem of cooperation versus conflict in groups. The JAVA applet demonstrates the spread of 'altruism' and 'exploitation for personal gain.'
   http://www.xs4all.nl/~helfrich/prisoner/

Iterated Prisoner's Dilemma  
   Game and simulation based on Prisoners Dilemma game theory as described in The Selfish Gene by Richard Dawkins: coded in Javascript by Wayne Davis.
   http://www.iterated-prisoners-dilemma.net/