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.
Trelby - The free, multiplatform, feature-rich screenwriting program!
Adobe Story - Adobe Story is a collaborative script development tool.
Scrivener - Scrivener is a content-generation tool for composing and structuring documents.
Slugline - Slugline gives back the screenwriter to a more established, more straightforward time when the words on the page started things out and last.