Olaf Quimby II
Olaf Quimby II may not have been exceptionally mad, for a Patrician, but he was an activist with some peculiar ideas which continue to shape Ankh-Morpork. He crusaded for literal truth and plain speech, accuracy and honesty in trade. He prohibited metaphors and similes which couldn't actually be proven. When Quimby learned that people were selling pork futures, he required them to build a warehouse for the pork-to-be. The Pork Futures Warehouse remains, as does a tendency to literal- (some say bloody-)mindedness in the city.
He died as a result of a test against a disgruntled poet to see if the pen really was mightier than the sword. In his memory, the axiom was officially amended to include the phrase "...only if the sword is very very small and the pen is very very sharp."
A small and specialised museum owes its existence to this ruler. It contains a bucket of Pitch (As Black As), the original Horse You Rode In On, and sundry other exhibits.