Apple iWork - iWork is an office suite by Apple.

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.

Airborn - Create and edit documents online without sacrificing your privacy

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