The following sites provide a great deal of useful information on postmodernism:
Mary Klages' Postmodernism Page. This page provides a useful breakdown of the relationship between modernity and postmodernity as well.
Postmodernism and Its Critics. Though focusing on anthropology, this page defines key concepts and introduces key theorists.
Contemporary Philosophy, Critical Theory, and Postmodern Thought. This page provides links to information on key postmodern thinkers.
Berkeley's Postmodernism Page. Provides great author links.
Georgetown's Po-Mo Page. A helpful breakdown of the characteristics of modernism and postmodernism.
John Lye's Postmodern Literature Page. A good breakdown of postmodernism as applied to the study of literature.