Okular - Okular is a universal document viewer based developed by KDE.
Scrivener - Scrivener is a content-generation tool for composing and structuring documents.
Sumatra PDF - Sumatra PDF is a slim PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI viewer for Windows.
Celtx - Celtx is a scriptwriting software platform with applications in a wide range of mediums but that specializes in helping screenwriters.
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.
Trelby - The free, multiplatform, feature-rich screenwriting program!