Today, David Cass is celebrated at the University of Pennsylvania. He was not only a supreme theorist, but also a driving force in the department, both with graduate students and with CARESS, a research center he founded to help researchers disseminate their work.
His work on general equilibrium theory is probably the best known. For example, he modified the Ramsey model to take into account discounting, and the model is nowadays often refered to as the Ramsey-Cass-Koopmans model. David Cass has also developed crucial analysis of the overlapping generations model and sunspot equilibria. He also advised numerous graduate students.
For people outside these circles, he will however be best remembered for his fight against the UPenn administration that tried to remove him from administrative responsibilities because he happened to be in a relationship with a (willing) graduate student. In reaction, he gave a memorable speech on women (pdf) that is a very good showcase of this free thinker.