Screenwriting software helps the user to format and present a screenplay properly. Apart from these specialized screenplay software applications, there are many add-ons or templates for popular word processors like Microsoft Word. These software packages adhere to various formatting and presentation conventions which are considered standard in Hollywood. Some are commercial products which must be purchased, others are shareware or even freeware. Others are web applications and can be used inside a web browser.
Sophocles - commercial story-development, scriptwriting and scheduling package (discontinued)
Story Touch - Professional screenwriting software for Mac OS and Microsoft Windows. Story Touch is a complete software for dramatic development, allowing you to write and analyze your script at the same time. It has a free version.
Rough Draft - A discontinued but useful free software with the standard formatting for writing scripts.
Writer Duet - A free real-time collaborative screenwriting program that lets any number of writers (privately) work on a script, immediately seeing all edits. Converts from/to other common screenplay formats.
www.ReadThrough.com - Website that instantly performs scripts for free, so they can be listened to from anywhere. Users can share scripts, get notes, add music/sound effects and images, and convert between various screenwriting formats.