AbiWord - AbiWord is a free software word processor.
Scrivener - Scrivener is a content-generation tool for composing and structuring documents.
LibreOffice - Writer - Open-source cross-platform word processor.
Celtx - Celtx is a scriptwriting software platform with applications in a wide range of mediums but that specializes in helping screenwriters.
Zoho Writer - Create. Review. Distribute
Trelby - The free, multiplatform, feature-rich screenwriting program!