Trelby - The free, multiplatform, feature-rich screenwriting program!
Scrivener - Scrivener is a content-generation tool for composing and structuring documents.
WriterDuet - Collaborative screenwriting for everyone* *Great for solo writers as well!
Celtx - Celtx is a scriptwriting software platform with applications in a wide range of mediums but that specializes in helping screenwriters.
Fade In - Fade In Professional Screenwriting Software is a complete application for writing motion picture...
KIT Scenarist - Simple and powerful application for writing screenplays.