The Internet now contains a massive number of sites devoted to the art and craft of screenwriting. The best place to locate these resources is to use one of the Media Links Sites listed in the online resources section of the Internet Filmmakers FAQ.
A couple of good starting points…
Screenwriters Resource CentreCharles Deemers Screenwriters Homepage
The Online CommunicatorDone DealScreenTalkEuropean screenwriters might find the EuroScreenwriters site of interest. The site features a writer CV database – with currently over 80 writers from 20 European countries listed – as well as an ezine with articles and interviews, and a unique European film database.
If you are a writer or producer of short screenplays, then Short Scripts Online should definitely be a port of call. This site provides writers with an opportunity to market their work, and producers with the tools to browse hundreds of short film script ideas and contact their writers.