Bibisco - Novel writing application with additional tools to aid in character development and world building, organizational featues, and the capability to anaylize your novel and provide helpful data at a glance.
Manuskript - Software for storytellers who are also planners, offering a detailed, hierarchal outliner and "novel assistant" to help you think it all out before you've started writing, as well as a distraction-free text editor.
Kit Scenarist - Cross-platform screenwriting application with a handy research module for easy reference of research material and virtual cards for mapping out your story structure.
GitBook - Software that takes advantage of Git's version control system for technical writing and documentation.
Twine - Tool for developing non-linear, interactive fiction.
Lyx.org - LyX is a document processor that encourages an approach to writing based on the structure of your documents (WYSIWYM) and not simply their appearance (WYSIWYG).
Text Editors
ghostwriter - Text editor that allows you to use Markdown to create documents in various formats, all within a clean and customizable interface.
Emacs - Advanced, power-user text editor with content-aware editing modes, built-in documentation, and much, much more.
Spacemacs - Alternative to Emacs with a VIM-like featureset incorporated into the editor.
espanso - A "text expander" that allows you to code custom keywords to make your typing more efficient.
Notebooks
Standard Notes - free, secure note-taking app with end-to-end encryption and cross-platform syncing on unlimited devices.
Joplin - Cross-platform note-taking app with end-to-end encrypted note syncing between devices and a handful of other great features.
LibreOffice - A truly free and complete office suite for everyone.
Collabora Online is a self-hostable online version of LibreOffice with collaborative editing. It needs a document management platform such as Nextcloud - many Nextcloud providers offer some version of Collabora Online.
OnlyOffice - Modern, full-featured Office alternative for collaborators. Excellent compatability with Microsoft Office documents and can also integrate with Nextcloud.
CryptPad is a self-hostable, cloud-based office solution that integrates OnlyOffice tools in the web browser, similar to Office365, and includes other features like Kanban and a real-time whiteboard aapplication.
Editing
LanguageTool - Grammer, style, and spell checker in the form of a desktop app, webapp, browser add-on, office-plugin, and more. Supports several common languages and counting.
Translation
LibreTranslate - Machine translation API for text and documents that is self-hostable, can be used offline, and has a free webapp.
Publishing
LaTeX - Document and book formatting software for anything from scientific research papers to high fantasy novels.
Scribus - Versatile desktop publishing toolset for finalizing print-ready books, magazines, and more.
gscan2pdf - Easily make PDFs or DjVus from scanned documents.
Sigil-Ebook.com - designed to make it easy to create great ebooks using the EPUB format.
Other
FreedomToWrite.org - Freedom to Write is a movement dedicated toward creating and supporting Free and Open Source software solutions for the writing industry.
WriterDeckOS.com - Convert any laptop and most chromebooks into a writer deck. A device designed solely for writing.