The Basics of Screenwriting

FilmmakerIQ mentions AFI’s The Basics of Screenwriting (hosted on Fathom):

"In this seminar from the American Film Institute, Amy Dunkleberger guides both novice and professional writers through the creation of an engaging screenplay. The seminar clearly and concisely leads you through the creation of an effective premise, compelling characters and a structure for your work. Throughout the sessions, writing techniques, presentation do's and don'ts, screenplay terminology and film clips juxtaposed with their shooting scripts help you develop your initial whim into your final screenplay."

FilmmakerIQ has related posts too (scroll down), like What’s Wrong With The Three Act Structure? -- and of course there's news & analysis from John August and Mystery Man on Film, and more on Wikipedia (Pre-production, Screenplay, Screenwriting) & all over.

There's already a market filled with script-oriented apps like Celtx (open source pre-production software), Final Draft, Gorilla, and others, as well as the forthcoming Adobe Story.

Also, AFI has short intro videos on filmmaking at ScreenNation Learn. Here's Scriptwriting (AFI's Lights, Camera, Education!):