Final Draft - Use your creative energy to focus on the content; let Final Draft take care of the style.
Celtx - Celtx is a scriptwriting software platform with applications in a wide range of mediums but that specializes in helping screenwriters.
Scrivener - Scrivener is a content-generation tool for composing and structuring documents.
KIT Scenarist - Simple and powerful application for writing screenplays.
Arc Studio Pro - Screenwriting just became easier
WriterDuet - Collaborative screenwriting for everyone* *Great for solo writers as well!