The Roku Channel is an aggregator of new and existing no-cost content.
Pluto TV began life as a live streaming service that offered multiple "channels" of content.
Pluto TV
Tubi TV: Offers a wide variety of movies and TV shows, including many popular titles.
Freevee
Formerly known as IMDb TV, Freevee is part of Amazon's streaming business.
Hoopla is a digital-media service that allows you to check out all kinds of stuff -- including movies.
Hoopla
Kanopy is a commercial-free streaming service that offers a large selection of indie films and documentaries.
VUDU
Vudu has been consistently expanding its free, ad-supported section, which lets you choose from a generous selection of Hollywood movies (mostly older titles, alas) to watch at no charge.