Not all screenplays are commercially published and it may be very difficult to locate the script for any given film, but the sites below, while not comprehensive (or, perhaps, strictly legal), provide film scripts in a variety of forms.
The Daily Script
"a collection of movie scripts and screenplays to serve as a resource for writers and actors and those who simply enjoy reading movie scripts. The movie scripts are presented in proper script format (for the most part) and where possible, multiple drafts are presented." (site)
Movie scripts online; includes drafts, transcriptions of dialog, etc.
The Screenplay Database
Contains 400+ screenplays in pdf format.
The Script Lab
"Screenplays are only made available to download by The Script Lab when they are done so by their copyright holders and available free of charge." (site) Include scripts for many films released from 2012-2015.
A database of hundreds of downloadable scripts, movie scripts, screenplays, and transcripts of current, classic and maybe a few soon-to-be-released movies, television, anime, unproduced and radio shows.
Internet Movie Database.
This database provides basic information about films, including director, cast, characters, ratings and reviews. It is probably one of the best film resources available on the Web.
Voice of the Shuttle/Media Studies
From UC-Santa Barbara, a compilation of Web links from a variety of institutions and organizations dedicated to the study of film, video, and other areas of mass culture, as well as to Web sites that focus on particular genres, directors, actors, theories, national cinemas, and historical periods.
"strives to be the most complete Internet directory of relevant and current movie-related web sites. Every link is reviewed by a live person and we work hard to make sure we only list active links with up-to-date content." [from the website]
Current film news, trailers, interviews, etc..
Video Detective- Streaming movie trailers