Most well known as the childhood home of Samuel L. Clemens (aka Mark Twain) and the inspiration for the fictional town of St. Petersburg in "The Adventures of Tom Sawyer" and other Twain books, Hannibal was founded in 1819, about 25 years before the Clemens family settled there.
Thanks to its prime location along the Mississippi and its importance to the lumber, rail and cement industries, the Missouri town grew in size from about 30 people in 1830 to some 2,000 by 1850. Today, the population is about 17,000 or so. Tourism is one of the city's main economic drivers, and there's a thriving arts community, as well.
Because of its relevance to Mississippi River history, the port is an important stop on Upper Mississippi cruises. A day in Hannibal takes visitors back to the mid-1800s; original building facades have been maintained or recreated so it isn't difficult to picture the town during its riverboat glory years. Samuel Clemens' original home, the fence Tom Sawyer convinced his friends to whitewash, the original Becky Thatcher home and a recreation of the "real" Huck Finn's home are all open for visitors. And don't be surprised to see at least one boy dressed up as young Tom Sawyer. (During the Tom Sawyer Days Festival, held over July 4th, gaggles of kids compete to be the town's main characters.)