Guide to Philosophy on the Internet - Very comprehensive, browsable.
Stanford Encyclopedia of Philosophy - Currently incomplete, but what's here is very good.
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.
Perseus - THE online place for ancient philosophy texts
"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.
Individual philosophers such as David Chalmers, Ned Block, and Hubert Dreyfus 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.