A guy named Rob has put almost all of Twain's works online. You can read Huck Finn, Tom Sawyer and many other fiction and non-fiction writings. Thanks, Rob!
Mark Twain's speech at his 70th birthday party.
In 1885 Twain designed a game to improve the memory. You can play it online here.
A PDF view of Twain's 1910 obituary in the New York Times.
E-text of Albert Bigelow Paine's official biography of Twain.