Check out this list of public domain films compiled by the Library of Congress

INFO-DIGI maintained a list of likely public domain films. Though the list is not comprehensive, it's a great place to start for an overview of many of the films that have entered public domain over the years. As always, if you plan on re-using any of the materials, do your due diligence to be sure the film is in fact in public domain.

Festival Films also maintains a list of films that have entered public domain over the years. Though they are a retailer, this list is good to consult for some information.

Public Domain movies has a list of many movies in public domain, with some watchable titles.