Well, this is a new one on me. As you may or may not know, when serving as President of the U.S., both George Washington and John Adams actually lived in Philadelphia in a mansion that was then the biggest in town. It was the precursor to Washington, D.C.'s White House -- and though it no longer stands -- its location has been identified.
What's really cool -- in this geek's mind, at any rate -- is that there's a 24/7 camera aimed at the excavation of the site. Check out archaeologists at work here. Just one word of advice -- I discovered this site (thanks to a tip from John Logan) in the wee hours of the morning, and there's not much action then. Daytime hours are slightly more interesting for the presidentially curious.