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