There are a reasonable number of film scripts available on the Internet now, however beware that many of these are not in the industry standard format. Problems with copyright have prevented more wide-spread availability of scripts, added to the fact that someone usually has to type them out word for word.
Probably the best place to start looking is Drew’s Script-o-Rama which contains archives and pointers to a large number of scripts on the Net. One thing I like about this site is the author has actually taken the time to classify which scripts are in the correct format and which are not.
ScriptCrawler is another service which provides links to scripts around the Net, and their archive is quite large now. And as the name suggests, Short Scripts Online is your source for shorter screenplays.