I've never actually written a script but I have been tempted and here are some links that I accumulated. The celtx link is to some free script writing software that I've heard good things about.

http://www.scifiscripts.com/default.html
http://www.thescriptorium.net/index.html
http://www.script-o-rama.com/oldindex.shtml
http://www.hcdonline.com/
http://www.hollywoodlitsales.com/
http://www.scriptassist.com/
http://www.celtx.com/
http://www.scriptfrenzy.org/
http://www.wordplayer.com/
http://www.scriptforsale.com/

Dave K