Overleaf - The online platform for scientific writing. Overleaf is free: start writing now with one click. No sign-up required. Great on your iPad.
AbiWord - AbiWord is a free software word processor.
TeXworks - The TeXworks project is an effort to build a simple TeX front-end program (working environment)...
LibreOffice - Writer - Open-source cross-platform word processor.
Archimedes - Archimedes is a LaTeX and Markdown editor designed to make mathematical writing easy.
Zoho Writer - Create. Review. Distribute