|Mark Twain In His Times: An Electronic Archive
This is the site I've been building since 1996, with resources from the Barrett Collection, UVA Special Collections.
|Mark Twain Project Online
This site is being built by the Mark Twain Project at the University of California, Berkeley. It has a lot of material about Huck Finn, but is especially valuable for its searchable collection of over two thousand Sam Clemens|Mark Twain letters, written through 1880.
|Mark Twain Quotations, Newspaper Collections, & Related Resources
This rich site is the work of Barbara Schmidt, featuring a quotation from MT for every occasion, scores of contemporary newspaper articles by and (mainly) about MT, and many great graphics from the collection of Dave Thompson.