Grammarly - Clear, effective, mistake-free writing everywhere you type.
Overleaf - The online platform for scientific writing. Overleaf is free: start writing now with one click. No sign-up required. Great on your iPad.
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
TeXstudio - TeXstudio is an integrated environment for writing LaTeX documents.
Editsaurus - Highlights potential pitfalls in your writing to encourage good habits.
LyX - LyX is a document processor.