General Resources:
Wga.org - Agent list, resource, articles, advice and interviews.
Scriptshack.com - Great script resource.
Screentalk.org - Scripts to read online.
Simplyscripts.com - Getting scripts sold.
Sagindie.org - Tons of great resources/info for indi filmmakers and writers.
Story Structure/Writers Toolbox:
Truby.com - One of the leading story structure methods, lots of product to buy as well.
Mckeestory.com - Another leading story structure method, lots of product as well.
Writersbootcamp.com - Leading writing process methodology, proven writing tools for screenwriters.
Writersstore.com - Software, DVDs, classes, writing supplies, caters to screenwriters.
Finaldraft.com - Leading screenwriting software.
Screenplay.com - Movie Magic screenwriting software (formerly Screenplay Systems).
[More links to come...]