Below are a select group of authoritative sites. With every site on the web, BE CAREFUL: there is very little decent textual criticism out there that you can trust. This is in part because copyright restricts the re-publication of authoritative studies: all of that information in all of those books and articles in the library is NOT on-line!
I URGE you as strongly as I can to avoid ".com" sites targeted at students. Their interest is not to bring you good or even accurate information, but to get hits to make adverstising dollars. Consider the source.
Metapages
The Voice of the Shuttle: Anglo-Saxon and Medieval is one of a set of VOS meta-pages for the humanities. The Voice of the Shuttle: Renaissance and 17th Century. The Orb is an on-line encyclopedia with refereed (hence, authoritative) articles on a number of topics for medieval studies.
Specific topics in Medieval Literature and Culture
The Internet Medieval Sourcebook has lots of great resources, primary and secondary. The Lollard Society website has a bibliography and many, many links to connect to sites on later medieval and Reformation religious culture. The Geoffrey Chaucer Page is the best site on Geoffrey; see my list of Helpful Websites for Chaucer for more if you like. Here is great information on Margery Kempe; it includes an excellent introduction and a link to the full (and fully glossed) text.
Specific topics in Early Modern Literature and Culture
Ballads and other pop music: here's a page on Sixteenth Century Ballads; and here's the Bodleian Library Broadside Ballads page. The Edmund Spenser Homepage Mr. William Shakespeare on the Internet is the best site on Shakespeare; also see my list of Helpful Websites for Shakespeare for more decent options. Here is the King James version of the Bible. The Milton-L Homepage, and the Milton Reading Room.