Links

Unions

Writer's Guild of America, West: The place to go for union issues in the USA and to register your script
Writer's Guild of Great Britain: The place to go for union issues and a cup of tea

John August: A ton of useful ideas from a pro - writer of "Go" and "Big Fish" among many others
Jane in Progress: Jane Espenson is a TV writer, currently working on the Battlestar Galactica spin-off "Caprica". She's retired her blog now but the archive is well worth checking out for her thoughts on what pilot scripts to write, what stale jokes to avoid and how to spice up your action lines.
Writing for Performance: Robin Kelly always has his ear to the ground. A great place to come for interviews with writers, and UK-based writing opportunities.
James Moran: James Moran's blog is always worth reading. Start with this astonishingly comprehensive "Big Writing FAQ".

Final Draft: The industry standard and not expensive.
Movie Magic: The only serious rival to the above. Less well-known and more expensive, but its users say it's wonderful.

In general we recommend you stay away from software which offers to do the actual writing for you!

How to lay out a screenplay: Software like Final Draft takes care of this for you and there are as many subtle variations on this basic template as there are screenwriters but this guide gives you the basics in a fairly clear way.
BBC Writers Room: Full of advice, templates, sample scripts, opportunities and ideas.
Wordplayer: Genius site run by Ted Elliot and Terry Rossio (Pirates of the Carribean / Aladdin) full of wit, insight and practical industry advice.

Impro by Keith Johnstone: The book that made Mark Ravenhill into a playwright (among many others).
The Improv Handbook: More thoughts on spontaneous storytelling, written by two of the script surgeons.
Story by Robert McKee: Solid advice, but doesn't pretend to be an infallible template (pace Charlie Kaufman)