Orestes Pursued by the Furies (1862)
Adolphe-William Bouguereau (1825-1905)
Chrysler Museum of Art, Norfolk, Virginia

An essential site for information about, and images of, anything related to ancient Greece is the Perseus Digital Library. Wonderful. Check out the very useful search facility. (If it's down, there are mirror sites in Oxford and Berlin.)

Robin Mitchell-Boyask has a Study Guide and structural charts of the Agamemnon, Libation Bearers, and Eumenides.

Michael Webster has background and questions and a summary of The Libation Bearers, as well as some background on Greek Tragedy in general.

Andrew Wilson has helpful pages about the Oresteia's themes and plot, the relation between free choice and fate, and some notes on the first chorus.

There are some online materials for a classical mythology text by Stephen L. Harris and Gloria Platzner. These materials include some online quizzes (with automatic grading); these are sort of fun despite having a lot of typos. There are also links to other online material, a few of which I've borrowed below.

A theater history site has a summary and analysis of the Oresteia, and some biographical material on Aeschylus. Both are from a 1906 book by Alfred Bates.

More biographical material.