As part of the Tribeca Film Festival, there's a drive-in setup on Pier 25 (West Side Highway at N[athanial (not orth)] Moore Street) and they'll be showing movies Thursday, Friday and Saturday night. My childhood favorite Grease will be shown Saturday. Details about tickets, films, and start times are here. All shows are free, but you must have a ticket to enter. I jogged by this morning and they have a movie marquee at the entrance and everything, it looks fun.