The Internet Archive has really done it this time: They have made available for your unrestricted downloading (or streaming) and enjoyment one of the undisputed classics of German moviemaking.
Behold the Fritz Lang masterpiece M: Eine Stadt sucht einen Mörder(1931), also known simply as M.
It is presented here with embedded English subtitles — a wise move.
Just calling it "a highly structured and stylized film about a serial killer" doesn't really do it proper justice, though.
May this one haunt you for days on end.