Final Draft - Use your creative energy to focus on the content; let Final Draft take care of the style.
Scrivener - Scrivener is a content-generation tool for composing and structuring documents.
Trelby - The free, multiplatform, feature-rich screenwriting program!
Masterwriter - MasterWriter offers a suite of writing tools.
KIT Scenarist - Simple and powerful application for writing screenplays.
WriterDuet - Collaborative screenwriting for everyone* *Great for solo writers as well!