I found a listing of 500+ quality films that you can watch online. The collection is divided into the following categories: Comedy & Drama; Film Noir, Horror & Hitchcock; Westerns & John Wayne; Silent Films; Documentaries, and Animation.
Here is the link to the page:
http://www.openculture.com/freemoviesonlineHovering over the word "Free" in each entry shows you where you can go to watch it. I have to assume most of these are public domain (good Rifftrax fodder perhaps?) but some are hard to believe. Yellow Submarine, really?