Existentialsim: A Primer Includes biography, chronology (in table form), bibliography, and summary of works for a number of thinkers including Heidegger and Sartre.
Hippias: Limited Area Search of Philosophy on the Internet: The most complete place to begin searching for information in philosophy.
Guide to Philosophy on the Internet: Maintained by Peter Suber, Philosophy Department, Earlham College, this is the best one stop shopping source for links to everything. Clearly arranges links by Guides, Philosophers, Topics, Associations, Journals, Teaching/Learning, Etexts, Bibliographies, Mailing Lists, Newsgroups, Projects, Preprints, Jobs , Dictionaries, Quotations, etc. Be warned-it is one huge page. ( There is also a multiple-file edition that loads more quickly, but requires a good bit of surfing to get around.)
Philosophy in Cyberspace: A very nicely, neatly laid-out site with links.
Russell: The Bertrand Russell Archives.
Heidegger: Existentialism and Martin Heidegger with biography, bibliography and a number of links.
Wittgenstein: The Wittgenstein page includes links to biographical essays, books, and Papers, Essays, Lectures, and Books Online.
Ayer: The Internet Encyclopedia of Philosophy article on Logical Positivism. A thorough, if technical, discussion with no links.
Sartre: A general website with links, etc.
Quine: A site maintained by his son, Douglas Boynton Quine. Includes a lot of textual information with links. An all-in-one lengthy webpage.
Derrida: Writing in Reserve: Deconstruction on the Net, Jacques Derrida Online." A deconstructed site.
Derrida: Resources related to Jacques Derrida and or Deconstruction on the Internet divided into the following sections: Email, Forums, Journals, Papers, Snippets, Book Reviews, Link Lists, Web Sites, Bibliographies, Derrideans Courses."