Stanford University
Ancient Skepticism/stanford Encyclopedia of Philosophy
An excellent, detailed, and lengthy overview of the ancient Skeptic tradition. Divides into Pyrrhonic and Academic Skepticism. Includes many quotes and a full bibliography as well. A superb treatment of the subject.
Internet Encyclopedia of Philosophy
Internet Encyclopedia of Philosophy: Greek Philosophy
Introduction from the Internet Encyclopedia of Philosophy to the main topics and thinkers in the Greek philosophical tradition. Includes links to related articles. Helpful for relating particular philosophers to each other chronologically.