This list is adapted from Literature: An Introduction to Fiction, Poetry, Drama, and Writing, p. 1474.
This site is often a good starting point. Wikipedia may be helpful for understanding a particular work of literature and for finding other reference sites. Bear in mind, however, that because literally anyone can edit Wikipedia, it is generally not a reliable source for referencing in your own paper.
Library of Congress’s literary criticism list
This collection of web links has been pre-screened by the Library of Congress.
Internet Public Library
This site is provided by the University of Michigan.
Voice of the Shuttle
You can find lots of good stuff here.
This site is a portal to thousands of libraries around the world.