The ruins of St. Paul's is the most famous sight in Macau. It was built in the early 1600's, and destroyed by fire in 1835. The facade is all that remains. Since it is such a famous site here, the facade is maintained, and there are even stairs that you can take to a walkway behind it to look out windows. People like to toss coins from the walkway to the windowsills, so you will see a lot of coins there. Be sure to stop by here while you are in Macau.
One of my favorite photos here is looking at the ruins from a side street. It seemed odd that although the streets are right next to each other, one was packed and the other was almost empty.