Guide to Philosophy on the Internet - Very comprehensive, browsable.
Stanford Encyclopedia of Philosophy - Currently incomplete, but what's here is very good.
The Internet Encyclopedia of Philosophy - Many classic philosophy texts online here.
No Dogs or Philosophers Allowed - "Your one-stop shop for multi-media philosophy!"
The Philosopher's Lighthouse - Sort of Philosophy 101 online.
The Fallacy Files - Excellent general site on logical fallacies
The Skeptic's Dictionary - Good clear and complete definitions of terms used in pseudo-science and critical analysis of pseudo-science (confirmation bias, Forer effect, etc.)
Index of Skeptical Inquirer articles online - Very useful for term papers in Critical Thinking!
Dr. Simanek's home page - A truly wonderful, comprehensive collection of links to sites on history of science, paranormal, creationism, flat earth, etc. Thank you, Dr. Simanek!
History of Science - From PBS. Many links.
Logic and Reasoning: Introductory Material - The host site is "exuberantly atheist".
Philosophy Songs - Requires MP3 capability for the full experience
Check out the The Internet Encyclopedia of Philosophy, which has links to many works online.
Perseus - THE online place for ancient philosophy texts
The Structure of Scientific Revolutions - A synopsis of Kuhn's very famous book
Voluntary Simplicity - Excerpts from the book of the same title
"The Will to Believe" - William James' famous essay
"The Fixation of Belief" - Essay by C. S. Peirce
Marx's Economic and Philosophical Manuscripts: Alienated (Estranged) Labor - Like the title says. Actually, pretty much everything by Marx is online from this site.
Descartes' Meditations Home Page - In French and Latin as well as English
Individual philosophers such as David Chalmers, Ned Block, John Searle, Hubert Dreyfus, and Daniel Dennett put current works in progress and archived papers on their Web sites. David Chalmers has compiled an excellent list of links to sites of this type.
Sure, you've read their books, but what did they look like?
Episteme Links Pics - Pics and links to other image databases.