The Writers Guild of America - The screenwriters union. Info on registering your work, rules for union writers and signatory producers, great research links, list of agents, contract forms and more. A good place to start.
Internet Movie Database - A "who's who" of the film industry. Listings by name, movie title, etc. Find info on films, credits, reviews, etc.
Script Secrets - A host of good links to screenwriting sites, with info ranging from legal and research to studio websites.
Roughdraft - A guy from the U.K. is offering a FREE, neat little word processing program for download. Not all the bells and whistles of the bigger, splashier programs like Final Draft or ScriptWare, but it writes in novel or screenplay format, and the price is sure right! Good starter program.
Stage Left Productions - Website for the Las Vegas producer of the indy film "Siren." Without doubt, the best indy production I've ever been involved with.
Drew's Script-O-Rama - My favorite site for acquiring screenplays. Listed alphabetically, you can download scripts to your heart's content, all for FREE!
Cinemarquee - More great links to screenwriting sites, resources, screenwriting software sites and more.
Colorado Film and Video Resources - Hosted by Inferno Films, LLC, local Denver producer. Contains a listing of local boards and commissions, film and video festivals, talent agencies, production companies, etc.
Screenwriting.info - Just what it says. This site addresses most of the technical issues of concern to aspiring screenwriters, such as formatting, style, etc.
The International Screenwriters' Association - An international resource for screenwriters worldwide.
"Writing is easy. All you do is sit staring at a blank sheet of paper until the drops of blood form on your forehead." - Gene Fowler