Carnegie Mellon's Books online. A clearinghouse for etexts. Usually very up-to-date.

Documenting the American South. UNC is scanning like crazy. Quite good for southern literature, but a great collection of autobiographies as well.

UVA's Electronic Text Center. A wickedly large collection of text, taken from all over the place. This site is sometimes slow. I think it's because they store many etexts as SGML and then spit it out to HTML on the fly. SGML is a preferable for storing texts, but the conversion seems to burn cycles.

American Memory Project. Thank goodness there is a Library of Congress. For those interested in the postwar US, the best collections so far are African-American Perspectives and The Evolution of the Conservation Movement.