ScriptWright turns Microsoft Word into a dedicated screenplay word processor. It contains all the styles, keyboard, menu and toolbar customizations, and tools to make writing and editing a feature screenplay a breeze.
Additionally, ScriptWright allows you to format your finished screenplay in three ways: Draft, Master, and Shooting.
CHAS_M If you already own Word, and don't want to shell out the bucks for Final Draft, Magic Movie Screenwriter or Montage, this might be a good alternative.
If you started your screenplay on a "friend's" copy of FD or MMS, you can now convert that file to and from ScreenWright's MSWord format, making this in my view now a complete product.
Haven't tried it yet, but you might want to check out the free trial -- I will. (Version 2007.9)