This site, hosted and organized by the University of Illinois's English Department, provides biographical information on Modern American poets as well as excerpts of scholarly criticism of their works. The site serves as a companion to the Anthology of Modern American Poetry from Oxford. It provides a good starting point for research on Modern American poets and their major works.
This in-depth bibliographic guide to American literature is maintained by English Professor Paul Reuben at California State University and is divided by time periods and authors and covers the 1700s to the late 20th century. It includes authors of poetry and fiction.
This website includes biographies of poets, sample poems, recordings of poets reading their works, teaching resources, articles on poets/poems, and many other useful poetry-related resources, including a complete archive of Poetry magazine.
Through Harvard University, the Woodberry Poetry Room's online "Listening Booth" provides streaming access to most of its audio recordings of poetry readings from a large number of prominent American and British poets including Robert Lowell, W. H. Auden, John Ashbery, and Amiri Baraka.