Edit: a lot of answers, thanks! But still, still kinda discussed. It certainly is an interesting topic.
But there are details you'd want to ask a lawyer about. Things like if you rip one of those movies from a DVD, and that DVD was made more recently, you have to be careful not to include any creative work done more recently - for example an adjusted end credits sequence, coloring, restoration work, or minor re-editing.
For truly free film, I believe you'd need to scan a print of the film. I'd be interested to know of any efforts to obtain and host truly free archival copies of these old films.
There's also Project Gutenberg and Standard Ebooks for public domain books. Wikimedia Commons has a huge archive of public domain images and sound recordings.
What about a derivative work? Like if somebody recreated the film Metropolis into a 3D version, scene by scene. Ultimately the same film but using Blender or whatever so every scene is perfect as a 3D movie. Is that new film copyrighted to the person who did this work now?
From https://en.wikipedia.org/wiki/Public_domain :
> Because those rights have expired, anyone can legally use or reference those works without permission.