I've lately come across a number of public-domain movies for which the rights have lapsed, that you can watch online. Some of them, while being low-budget, are wonderfully atmospheric, or just plain fun.
For openers, Ed Wood's
Plan 9 From Outer Space (1959)
I remember watching this one on TV when I was growing up in the early 70's, before it gained acclaim as "the worst movie ever made", and they came up with an annual award for "worst" films, the Golden Turkey awards.
I also love the 1994 semi-bioographical Tim Burton movie
Ed Wood, for which Martin Landau won an Oscar for Best Actor, for his portrayal of Bela Lugosi. A quirky fun movie that has the trappings of a horror/sci-fi movie itself, but hilariously portrays Ed Wood and his circle of actor-friends struggling to survive and make films, and also vicariously shares Ed Wood's enthusiasm for storytelling, despite his hardship and questionable talent. My favorite scene is where Ed Wood is walking by a funeral parlor, and sees Bela Lugosi (knowing that he hasn't much longer to live) trying out coffins. The idea of the actor famous for playing Dracula shopping for coffins, and irritably laying in them to try out each one!
Anyway, enjoy.
