Topic: Very useful free pre-production script writing software.
Straight off the bat I'd like to say I'm not being paid to advertise this product (I wish), I simply stand by it as and awesome and useful program.
Celtx is a script writing program, much like your average word processor, specifically for writing standard format screenplays. Each project also has a redrafting mode (edits are made in blue and can be reset if required), an annotation mode (for making notes) and a master catalogue where you can list your characters, actors and props, along with descriptions and details.
It's just awesome, check it out guys. 
http://www.celtx.com/
