Apple iWork - iWork is an office suite by Apple.
Scrivener - Scrivener is a content-generation tool for composing and structuring documents.
Airborn - Create and edit documents online without sacrificing your privacy
Celtx - Celtx is a scriptwriting software platform with applications in a wide range of mediums but that specializes in helping screenwriters.
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.
Trelby - The free, multiplatform, feature-rich screenwriting program!