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.
TeXstudio - TeXstudio is an integrated environment for writing LaTeX documents.
LibreOffice - Writer - Open-source cross-platform word processor.
LyX - LyX is a document processor.
Zoho Writer - Create. Review. Distribute