LibreOffice - Writer - Open-source cross-platform word processor.
Scrivener - Scrivener is a content-generation tool for composing and structuring documents.
Microsoft Word - Microsoft Word is a commercial word document processor for Windows.
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!