With HBO Max having launched in May and NBCUniversal’s Peacock looking to enter living rooms in July, the number of streaming platforms for consumers to choose from continues to multiply. It can be easy to get lost in that shuffle and forget that there are a vast number of films that are already available to watch online without any cost of admission at all. Services like YouTube and Tubi have quietly accrued brimming libraries with the only cost being an occasional ad break and a sturdy internet connection. These include classics that have entered the public domain, like Orson Welles’ “The Stranger” and Stanley Donen’s “Charade.” Some newer films are simply available through licensing agreements, parts of a cycle of free content to get to before they expire and are replaced by other films.

As these libraries continue to fluctuate, it’s easy to lose track of what...