I saw this on Collider.com and thought I'd share it here.  It an impromptu (well, for the library visitors at least) scene from the first Ghostbusters movie.  
Ghostbusters Return to the New York Public Library is the article title in case you want to read the particulars.   This qualifies as "YouTube spam", so I am keeping this off the main boards.

Hahaha pretty awesome video.
 
One of my favorites is the giant dance routine in the middle of that train station.  That one was so good.