As rough rule of thumb, feature films made in the US after 1923 are subject to copyright.  A number of  film producers and copyright owners have surrendered their rights and allowed their films to enter the public domain.

 Many of these films are available online and can be downloaded; some are available for sale, but because copyright no longer holds, performance and other rights may be more freely available than with copyright films.  Many film clips, short films, documentaries and other material may also be copyright free and in the public domain.  Even when materials are reproducible, copyright free or in the public domain, there may still be restrictions on how they may be used; advice is always to check the small print.

 The US has reciprocal agreements regarding copyright with most countries.  Local terms may differ, for example how long copyright lasts, but the basic rule is that a work copyright in USA is protected by copyright law in other countries.

 The Wikipedia material and the Wikimedia material look promising.  However, the Disclaimer pages point out that anyone can post anything in these wikis.  Any claim that they are copyright free and in the public domain is not necessarily true.  It remains the user’s responsibility to determine the exact copyright position.

 The above information is for guidance.  In all cases (not just wiki material), the exact copyright and reproduction position needs to be checked;  if copyright information is not clear then it may be necessary to contact the copyright holder to obtain permissions and/ or professional legal advice should be sought.

 Internet Archive: Moving Image Archive 
More than 31000 films and movies in the public domain or on open license, ranging from short films to feature films, television programs and advertisements, news clips and documentaries. 

 Library of Congress : American Memory : Origins of American Animation, 1900-1921
Includes a small collection of downloadable early animation films.

 Creative Commons  | Video: movies, animations, footage...
“Creative Commons helps you publish your video online while letting others know exactly what they can and can't do with your work.”  Restrictions often still apply.

 Common Content : an open catalog of Creative Commons licensed content
A catalog of works licensed in the Creative Commons; includes movies.

 Wikipedia : Public Domain Films    
“Articles about post-1923 theatrical-released films that are in the public domain in the region of original release, the United States, and/or other English-speaking regions. Media may be available on Wikimedia Commons or other sites such as the Internet Archive.
 “Due to the Copyright Term Extension Act, no more films will enter public domain in the United States until January 1, 2019, when films released in 1923 will be ineligible for copyright (and 1924 films in 2020, 1925 films in 2021, and so on).”

 Wikimedia Commons
A collection of media believed to be copyright free or in the public domain; includes moving images.

 NARA : National Archives
Free online access to more than 100 historic films, newsreels and documentaries.

 CandyJar
The Candy Jar is a collection of video clips produced mainly by VJs for VJs. The clips are licensed for non-commercial use - you'll have to read and agree to the licence when you download.

 OpenFlix Public Domain Movies 
“OpenFlix provides a directory of movies commonly thought to be in the public domain and works their owners are willing to let be distributed."
“Public Domain refers to the body of creative works and knowledge in which no person, government or organization has any proprietary interest such as a copyright. These works are considered part of the public cultural heritage and may be freely used by all."
“Use of movies in the public domain, though, is limited by difficulty of identifying works that truly do not have any proprietary interests. Through dialogue, OpenFlix hopes to resolve some of the confusion.”

 Festival Films : Public Domain 
Commercial site which sells films believed to be in the public domain.
DEFINITELY PUBLIC DOMAIN
“The lists that follow contain mostly American films made before 1964. These films are undisputedly in the public domain in the USA. In most cases no underlying music or literary rights have been claimed. If we know of any claims we will inform you before proceeding."
MULTIPLE USES
“These films may be broadcast on cable TV, sold on VHS or DVD, used for stock footage in various projects and shown to audiences where admission is charged. They may even be dubbed into foreign languages!”

 Classic Movies - Streaming Video    
Links to sites which offer downloadable movies, clips, trailers, many for free, many for purchase.  Some require specific software.

 soccerclips.net :: Biggest online soccer videos archive  
Just what it says!

 British Pathe Digital News Archive  
Some clips for sale, many are free; intended for UK audiences but some material is available for out of UK use.

 BBC News Open News Archive 
Downloadable news clips (trial).  May be available only to UK residents.