Open Access Video By Type

Calisphere Collections from the California Digital Library; search by topic and use 'motion pictures' filter
EUScreen Thousands of items from the audiovisual heritage of the European Union
Internet Archive Over 6 million videos, most available for download and reuse
Library of Congress Video Search Over 9000 films available for free viewing
Med-Mem Audiovisual memory of the Mediterranean
Wikimedia Commons Eclectic collection of freely reusable video

Cinemargentino Recent Argentinian feature films, documentaries, and shorts; Spanish-language, some films have English subtitles
Cineteca Nacional de Chile Spanish-language films from Chile (scroll down to Cineteca Nacional Online)
Korean Classic Film Subtitled feature films spanning the history of Korean cinema (Korean Film Archive)
L.A. Rebellion Films Small collection of student films by Black filmmakers in 1970s Los Angeles
MosFilm Hundreds of films from the major Russian film studio of the Soviet era (not all films are available for viewing)
Movieclips Large collection of licensed clips from mostly Hollywood films
National Film Board of Canada: 3,000 documentaries, animations, experimental films, fiction and interactive works. Most of the films reflect Canadian interests; some films must be downloaded rather than streamed.
National Film Registry A small collection of films from the Library of Congress' list of important US films to stream or download
National Screening Room Films from the 1890s to the 21st century from the Library of Congress digital collections
Russian Film Hub Around 500 films with English subtitles; most films are free to watch
Thanhauser Films Silent cinema from an early US film company
The World Film Heritage, 1895-1915 Early silent cinema from Europe and the U.S.

Folkstreams Documentaries on the "culture, struggle, and arts of unnoticed Americans"
Media Burn Documentaries with a social justice lens; international in scope but mostly US-focused
Media That Matters Documentaries from the Media That Matters annual film festival
 

American Rhetoric Video of major American political speeches, sermons, debates, and more. Includes audio and transcripts
The Living Room Candidate Presidential campaign commercials 1952-2020 (Museum of the Moving Image)
 

AdViews Thousands of television advertisements created from the 1950s to the 1980s
American Archive of Public Broadcasting 1000s of hours of video from public television & radio broadcasts 
C-Span Video Library Archived content of C-SPAN programming going back to 1987
The Interviews: An Oral History of Television In-depth interviews with key television figures
 

American Indian Film Gallery Films documenting North American indigenous people, from 1922-2011
Civil Rights Digital Library Raw news footage and edited television broadcast from the Civil Rights movement; browse Media Types section to find moving images
Digital Moving Image Collections  Several hundred early motion pictures covering a wide range of subjects (Library of Congress)
Prelinger Archive Hosted by the Internet Archive, huge collection of ephemeral films (educational, industrial, home movies)
US Holocaust Memorial Museum Large collection of oral history interviews and more
WGBH Open Vault Online access to historically important content produced by WGBH
World Digital Library (Library of Congress)

CDC-TV Short videos from the Centers for Disease Control and Prevention on health, safety, and preparedness
NPS multimedia search Public domain films from the National Park Service featuring parks, historic sites, monuments, etc. For a small curated list see National Park Service B-roll of the Grand Canyon
ScienceCinema Content from the US Department of Energy, includes conference recordings, promotional videos, research recordings, interviews, and more
Southern Foodways Alliance Films Short films that explore "the diverse food cultures of the changing American South"
Wellcome Library YouTube Channel Medical history videos from the Wellcome Library

UbuWeb Encyclopedic collection of avant-garde video
Videoteca Spanish language videos on literature and theater from the Biblioteca Virtual Miquel de Cervantes
Video Data Bank Video by and about contemporary artists (School of the Art Institute of Chicago)

These two sites list archives and film institutes that have made content available for online viewing

Film Studies for Free
FIAF list of film archives with online content
___________
Open Culture Long list of films available to view online from a wide variety of sources

Ad-Supported
While the content on these sites doesn't require a financial transaction, you'll have to sit through advertisements. Like watching broadcast TV before DVRs.

Crackle
IMDB TV

Tubi

Freely Available Video

Important: While the content on these sites is free to watch, it is not necessarily available for legal reuse or sharing. Please check the rights and licensing pages carefully before doing anything other than watching the video for personal use.

AdViews Thousands of television advertisements created from the 1950s to the 1980s
American Archive of Public Broadcasting 1000s of hours of video from public television & radio broadcasts 
American Indian Film Gallery Films documenting North American indigenous people, from 1922-2011
American Rhetoric Video of major American political speeches, sermons, debates, and more. Includes audio and transcripts.
Calisphere Collections from the California Digital Library; search by topic and use 'motion pictures' filter
CDC-TV Short videos from the Centers for Disease Control and Prevention on health, safety, and preparedness
Cinemargentino Recent Argentinian feature films, documentaries, and shorts; Spanish-language, some films have English subtitles
Cineteca Nacional de Chile Spanish-language films from Chile (scroll down to Cineteca Nacional Online)
Civil Rights Digital Library Raw news footage and edited television broadcast from the Civil Rights movement; browse Media Types section to find moving images
C-Span Video Library Archived content of C-SPAN programming going back to 1987
Digital Moving Image Collections  Several hundred early motion pictures covering a wide range of subjects (Library of Congress)
Ensia Collection of videos on current environmental issues
EUScreen Thousands of items from the audiovisual heritage of the European Union
Folkstreams Documentaries on the "culture, struggle, and arts of unnoticed Americans"
Internet Archive Over 6 million videos, most available for download and reuse
The Interviews: An Oral History of Television In-depth interviews with key television figures
Korean Classic Film Subtitled feature films spanning the history of Korean cinema (Korean Film Archive)
L.A. Rebellion Films Small collection of student films by Black filmmakers in 1970s Los Angeles
Library of Congress Video Search Over 9000 films available for free viewing
The Living Room Candidate Presidential campaign commercials 1952-2020 (Museum of the Moving Image)
Media Burn Documentaries with a social justice lens; international in scope but mostly US-focused
Med-Mem Audiovisual memory of the Mediterranean
Media That Matters Documentaries from the Media That Matters annual film festival
MosFilm Hundreds of films from the major Russian film studio of the Soviet era (not all films are available for viewing)
Movieclips Large collection of licensed clips from mostly Hollywood films
National Film Board of Canada: 3,000 documentaries, animations, experimental films, fiction and interactive works. Most of the films reflect Canadian interests; some films must be downloaded rather than streamed.
National Film Registry A small collection of films from the Library of Congress' list of important US films to stream or download
NPS multimedia search Public domain films from the National Park Service featuring parks, historic sites, monuments, etc. For a small curated list see National Park Service B-roll of the Grand Canyon
National Screening Room Films from the 1890s to the 21st century from the Library of Congress digital collections
Prelinger Archive Hosted by the Internet Archive, huge collection of ephemeral films (educational, industrial, home movies)
ScienceCinema Content from the US Department of Energy, includes conference recordings, promotional videos, research recordings, interviews, and more
Southern Foodways Alliance Films Short films that explore "the diverse food cultures of the changing American South"
Thanhauser Films Silent cinema from an early US film company
UbuWeb Encyclopedic collection of avant-garde video
US Holocaust Memorial Museum Large collection of oral history interviews and more
Videoteca Spanish language videos on literature and theater from the Biblioteca Virtual Miquel de Cervantes
Video Data Bank Video by and about contemporary artists (School of the Art Institute of Chicago)
Wellcome Library YouTube Channel Medical history videos from the Wellcome Library
WGBH Open Vault Online access to historically important content produced by WGBH
Wikimedia Commons Eclectic collection of freely reusable video
World Digital Library (Library of Congress)
The World Film Heritage, 1895-1915 Early silent cinema from Europe and the U.S.

Aggregator pages:
These two sites list archives and film institutes that have made content available for online viewing
Film Studies for Free
FIAF list of film archives with online content
___________
Open Culture Long list of individual films and collections available to view online from a wide variety of sources

Ad-Supported
While the content on these sites doesn't require financial transactions, you will have to sit through advertisements. Like watching broadcast TV before DVRs.
Crackle
Tubi