Story Architect (STARC) - STARC - A smooth yet powerful screenwriting experience for devoted authors. With only wanted features and simple workflow it runs faster than all your previous apps combined.
Final Draft - Use your creative energy to focus on the content; let Final Draft take care of the style.
Trelby - The free, multiplatform, feature-rich screenwriting program!
Celtx - Celtx is a scriptwriting software platform with applications in a wide range of mediums but that specializes in helping screenwriters.
WriterDuet - Collaborative screenwriting for everyone* *Great for solo writers as well!
KIT Scenarist - Simple and powerful application for writing screenplays.