If you haven't yet had a chance to visit Paul Sahner's blog NYC Grid, now is a good time to start. He recently launched a "Before & After" feature in which he re-creates shots from the Library of Congress' collection of 20th century New York City photography--and then he binds those photos to his own with a "before and after" bar you can slide back and forth.
It gives you a momentary feeling of control over the city and its changing.
Slide the doohickey to the left and watch the city get boring as shoeshine men, neon signs, and well-dressed ladies vanish before your eyes.
Slide it to the right and watch the dull city of today magically disappear, replaced by puffing Camel signs, juice drink stands, and sailors on ice skates.
(Don't miss Paul's 1961 collection, too.)