The Bancroft Library at the University of California, Berkeley, has created an online exhibit called Mark Twain at Large: His Travels Here and Abroad. This site is a wonderful source of information about Twain's life and travels, and it has pictures of his early manuscripts.
The Mark Twain Official Home Page, created by the estate of Mark Twain, has more biographical information as well as a photo gallery and some of Twain's best quotes.
See some pictures of the house that Mark Twain lived in at the Mark Twain House website.