Mark Twain would doubtlessly enjoy the fact that he managed to set up his own website a century after his death. This site is maintained by the Twain estate and provides a solid introduction to the writer's life. It is also the place to go if you want to license Twain's words or image for some reason.
A truly amazing archive of primary documents. Want to see the original advertisements for the first edition of Huckleberry Finn? They're here. Want to see Mark Twain's signature? It's here. Professor Stephen Railton and the Electronic Text Center at the University of Virginia get major props for this site.
Mark Twain is often quoted (and misquoted), but this site does a fantastic job of archiving and organizing his many gems. His quips are organized alphabetically by subject, so that you can read Twain's zingers on everything from adultery to zephyrs. It also has an archive of his newspaper articles.
This is the recently-launched online version of the University of California's extensive archive of Twain's private papers. It's an exceptionally useful site that should only get better in coming years as more texts and resources are added. Among the many offerings are more than 2,300 of Twain's letters and biographies of virtually every major player in Twain's life.
This is the companion website to the 2001 documentary of Twain's life directed by filmmaker Ken Burns. Its coolest features are interactive versions of the many scrapbooks Twain kept throughout his life.
The house where Twain and his family lived in Hartford, Connecticut is now a museum dedicated to preserving Twain's legacy. Got some extra cash and want to contribute to literary history? The museum is raising money to restore the books in Twain's personal library.