Literary and Historical Shrine
Anyone interested in American Literature owes it to him/herself to visit this wonderful house. This is where many of Twain's most famous books were written, largely in the pilot-house shaped pool room on the third floor. The brick architecture is pretty nice too, though the interior has the usual Victorian darkness. Tour guide on our trip was excellent and vibrant. Great accompanying museum too.
If you visit, take in the Harriet Beecher Stowe house next door at the same time -- also highly recommended.