Youtube.com is a main source for video clips. There are many sources on Youtube, but on good starting place if you are looking for clips from feature films is the Movie Clips Channel
Clip Cafe is another source for clips. Many clips from 100s of movies available. Resolution is fairly low.
Links to some useful (and, in some cases, free) software for working with film clips:
There are several film scripts available in Infokat, the libraries' catalog, including several as part of the series "Newmarket shooting script series."