Okular - Okular is a universal document viewer based developed by KDE.
Final Draft - Use your creative energy to focus on the content; let Final Draft take care of the style.
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.
Scrivener - Scrivener is a content-generation tool for composing and structuring documents.