www.imdb.com -  A monolithic database containing detailed information on every film ever made.

www.script-o-rama.com  - Drew’s Script-O-Rama contains a huge index of movie scripts available for free.

www.writersstore.com - You supply the talent, this store supplies the rest

www.inktip.com - A great networking site where writers, producers and agents come together.

www.wga.org - Official sites of the Writers Guild of America, West and East. 
www.wgaeast.org

www.copyright.gov - The only way to completely protect your script is to copyright it.  A must site for every screenwriter.

www.creativescreenwriting.com - A great magazine for screenwriters

www.scriptmag.com - Another great screenwriting magazine

www.amazon.com - …because any book or DVD you really need you can find a used copy of here for peanuts.