The internet classics archive, an incredibly useful resource.
The text of the Oresteia is here in its entirety, and the site has an excellent search engine that can shave hours off of writing papers. If you are looking for a specific line or need to read every part of the trilogy that concerns Aegisthus, this is the place to go.
This site is about Homer,
but Aeschylus himself said that his plays were only the scraps left over from Homer. The Trojan War and its legacy is a vital part of understanding the Oresteia, and some knowledge of Homer is very helpful for anyone reading Greek tragedy. This site has valuable links for gathering information on Homer and the ancient Mediterranean. Not all of the links actually work, but be patient. There's valuable material here, as long as you're patient enough to find it. For starters, follow the link that says "Troy: Homer and the Troy Cycle." It's on the right hand side, near the top of the page.
"The Classical Origins of Western Culture."
http://depthome.brooklyn.cuny.edu/classics/dunkle/studyguide/studygde.htm
This site has basic study guides for a range of Greek and Roman texts. Especially helpful is the attention to the words in the original Greek: for those readers frustrated by the translation barrier, this study guide has sections that briefly explain and define some of the key words of the plays, giving us a small taste of the original language.
Mythological background of the trilogy.
http://www.usask.ca/antharch/cnea/CourseNotes/AgBckgnd.html
Very useful for information on the House of Atreus.
