Famous for that poorly built tower that's tipped over a bit. The Leaning Tower is the attraction. The tower is actually the bell tower of the adjacent Pisa Cathedral. It was built in the 1100s and began to tip almost immediately due to poor engineering.
Pro Tip: Not sure I would go out of the way to see this but if you're passing by or staying in Tuscany then sure, give it a day trip kinda look. Take the pic where it looks like you're holding it up. Have lunch in town. Take the train somewhere else.