Final Draft - Use your creative energy to focus on the content; let Final Draft take care of the style.

Google Docs - Create a new document and edit with others at the same time -- from your computer, phone or tablet. Get stuff done with or without an internet connection. Use Docs to edit Word files. Free from Google.

Celtx - Celtx is a scriptwriting software platform with applications in a wide range of mediums but that specializes in helping screenwriters.

Microsoft Word - Microsoft Word is a commercial word document processor for Windows.

Scrivener - Scrivener is a content-generation tool for composing and structuring documents.

Apache OpenOffice Writer - Open-source word processor of the Apache OpenOffice suite.