Granted, a lot of public domain movies are the same rubbish you’ll see flogged on substandard DVDs in $2 shops — but there’s also classic works from master directors like Alfred Hitchcock and genre-defining flicks like His Girl Friday. You can download films (as ever, be careful of your download limits) or watch them in a streaming player on the site.
The list of films sorted by popularity is one good place to start getting ideas, though the fact that the top-ranked title is called Sex Madness tells you a lot about our collective porn-loving ways. Thanks Dan W!
Internet Archive: Free Downloads: Feature Films