A few years ago, an editorial assistant at Random House named Rebecca Shapiro compiled a list of all the outdoor screenings of movies that happen in New York each summer. The list has become very popular with her friends, not to mention the public at large. This year, her list even got published in the NY Times.
You can check out this comprehensive guide here on the City Blog page. It's really handy to see them all together in one listing. There are so many these days! I just went through and marked my calendar with the one's I want to check out: top of my list are "The Goodbye Girl" in Bryant Park and "Starship Troopers" at McCarren Park. See you at the free movies!