I never see my the comments people make when they make them only noticing them weeks after the fact. Well, a Dave left me information about a NYC website called City Map. Using the clever sliding bar you can see aerials maps of the city from 1924, 1996, 2006 and 2008. You can slide back and forth switching the pictures. It's awesome.
All the big houses I've written about (Ward Mansion, etc.) are present in gloriously blurry black and white. It's the coolest thing I've been shown in ages. Have fun!