Who was Jack the Ripper?

In 1888, Jack the Ripper killed at least five women in London, mutilating their bodies. A number of letters, supposedly from the Ripper, were sent to police taunting officers' efforts to find the Ripper. (Whether any of them were actually written by the Ripper is a matter of debate among scholars.) The name "Jack the Ripper" comes from these letters. 

Needless to say, the Ripper was never found, and over the years, dozens of people have been brought up as possible candidates. A recent book suggested that a woman named Lizzie Williams was the Ripper, although other Ripper experts cast doubt on it. It appears unlikely that the true identity of the Ripper will ever be known for sure.