Abbot Management
Screenplay agency offering a great service in free feedback to all submissions.

BBC Writer's Room
Site on writing for TV, film and radio, complete with BBC guidelines.

Creative Screenwriting
Useful trade magazine for screenwriters.

Done Deal
Primarily a US site but full of resources for any screenwriter.

Drew's Script-O-Rama
Huge transcript site of film and TV scripts.

How To Format Your Screenplay
Excellent guide to screenplay formatting; essential reading!

Ink Tip
Highly proactive service to place scripts with producers, low fee (around £30) and quite a few successes.

The Movie Cliches List
Primarily entertainment but full of regularly fallen into traps.

Red Inkworks
Huge resource with hundreds and hundreds of excellent writing relevant links.

Screenwriting.info
Nice "beginners guide to screenwriting" sort of site with lots of detail on the basics.

ScriptWriter Magazine
UK based magazine writers magazine deserving of support.

Script Fly
Big resource site on screenwriting.

Shooting People
Legendary UK (and now US) resource based around a well subscribed mailing list; free to read, £30 per year to post.

Simply Scripts
As the title says another large transcripts site with some good links.

Sunset Script
Good script evaluation service who also promote good work to the industry.

Trigger Street
Kevin Spacey's well known community for new writers and directors.

Underground Film
Great site to post your work on with a simple submission process and plenty of different subjects and genres.

Wordplay
Superb resource full of articles and interviews on the screenwriting trade.

Writer's Copyright Association, UK
Simple and affordable protection similar to the Writer's Guild of America service.