The internet classics archive,
an incredibly useful resource. The text of Medea 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 play that concerns "love" or "exile", this is the place to go.
The Greek Mythology Link,
an incredible site. Well-designed, informative, and fun, the Greek Mythology Link is a tremendous resource for getting background information on Medea and other plays. Included are maps, biographies of important heroes and deities, and a search engine.
"The Classical Origins of Western Culture."
http://depthome.brooklyn.cuny.edu/classics/stdygde.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.
Links to summaries of myths.
This site is about Homer,
but all of the Greek playwrights worked in Homer's shadow. 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.