GIMP - One of the most popular Photoshop alternatives with an impressive feature set and versatility.
PhotoGIMP is a patch for GIMP that makes it much more like Adobe Photoshop for those that are accustomed to that experience.
Glimpse - Fork of GIMP with a more modernised UI, a more streamlined user experience, and a name with less ableist connotation. Keyboard shortcuts from PhotoGIMP are baked in. The project is no longer in active development, but a rewrite is in the works called "Glimpse NX" which is aiming to hone in even more on usability and accessibility.
Darktable - Virtual lightroom and darktable for photographers, allows for processing of raw image files.
DigiKam - Tool for sorting and organizing expansive collections of photos, with batch editing features.
Photoprism - AI-powered application for advanced photo organization and easy sharing.
pngquant - Efficient command line utility for compressing PNG files.
Krita - Advanced painting program for artists of any skill level. It's cross-platform, feature-rich, customizable, and intuitive to use. It even provides some photo manipulation and basic animation capability.
OpenToonz - 2D animation program geared toward traditional frame-by-frame animation with support for plug-ins that utilize deep learning to add impressive visual effects, based on software adopted and customized by the incomparable Studio Ghibli.
KumoWorks is a cloud rendering tool for OpenToonz animations
Synfig Studio - 2D Animation software that appears to be no longer actively maintained, though it is feature-rich and provides support for bone rigging, cut-out animation, and traditional frame-by-frame.
Papagayo-NG - Tool for generating 2D lip sync animation. The result typically needs some manual tweaking, but it takes a lot of work out of the process. It works with Synfig, OpenToonz, and Krita, among others.
Storyboarder - Storyboard and animatic software for easy visualization of your film/animation project during the pre-production phases.
RenderChan - Rendering manager for 2D animation projects.
OBS Studio - A brilliant piece of software for livestreaming and video recording.
kdenlive - Video editor with multi-track capability, support for a wide varitey of video and audio formats, and a suite of useful production tools. It's been in active development since 2002!
Shotcut - Video editor with professional audio processing features, lots of video effects, and a customizable interface with undockable panels. It also boasts support for some video-related hardware.
Pitivi - Video editor with a focus on design and usability, with natural integration into the GNOME desktop environment.
Flowblade - Video editor with a multitude of included tools, boasting stability and a speedy workflow.
Olive Video Editor - Video editing software with node-based compositing built-in and fast disk cache performance.
OpenShot - Simple, bare-bones video editing akin to Windows Movie Maker or iMovie.
Avidemux - Small but mighty. Ideal for quick edits, filtering, and encodes.
OpenColorIO - Color grading/management tool for use with Olive Video Editor, Blender, Krita, and more.
Natron - Professional compositing software for Motion Graphics and VFX.
Handbrake - Powerful, cross-platform video transcoder.
FreeCAD - 3D parametric modeler for product design, mechanical engineering, and architecture. Features Python integration, OpenSCAD integration, and 2D drafting.
OpenSCAD - Code-based, non-interactive 3D modeling workbench.
OpenVSP - Parametric aircraft geometry tool for designing functional air vehicles.
Avogadro - Molecular editor and visualizer for chemical engineers.
MolView - Web-based molecular editor and visualizer.
Tenacity - Simple, intuitive multi-track recorder and waveform editor. A more FLOSS-friendly fork of Audacity, albeit no longer maintained.
Audacium - Another Audacity fork, also no longer actively maintained, though the maintainer is looking to re-home the project.
Music Source Separation Libraries
Open-Unmix - CLI program that will attempt to separate individual tracks from an audio recording using PyTorch.
Spleeter - Another music source separator developed by Deezer developers.
Demucs - Yet another, but developed by Meta researchers (I know, I'm sorry, but it is entirely FOSS).
Digital Audio Workstations
Ardour - Full-featured, cross-platform, multi-track workstation & MIDI sequencer with support for VST, LV2, and LADSPA plugins. Best option for working with audio tracks.
Qtractor - Multi-track recorder/editor with a powerful MIDI sequencer for Linux. Supports VST, LV2, DSSI, and LADSPA plugins. Not as capable as Ardour in working with audio tracks, but much better at handling MIDI tracks.
LMMS - Ideal workstation for beatmaking or a loops-based workflow, like you may find in FL Studio or Ableton Live, though it does not feature native audio recording.
Ensembles - Digital workstation for arranging music for live performance, not targeted toward studio recording.
Calf Studio Gear - Extensive pack of LV2 plugins including instruments and effects in modulation, delay, dynamics, EQ, and more.
Zam-Plugins - Several audio plugins including a really nice dynamic EQ and more, all in LV2 or VST formats.
DPF-Plugins - Small collection of plugins by DISTRHO in LADSPA, DSSI, LV2, VST2 and VST3 formats.
DISTRHO Ports - Collection of open-source plugin ports to Linux, including the popular TAL-Noisemaker, Vitalium, mverb, and more.
HUGETracker - Tracker for Game Boy homebrew and chiptune composers.
FamiStudio - Tracker for NES/Famicom homebrew and chiptune composers.
GoatTracker 2 - Tracker for Commodore 64 homebrew and chiptune composers.
Mixxx - Community-driven software with dozens of features for newbie DJs and master turntableists alike.
MuseScore - Create and print your own sheet music.
Specialized Word Processors
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.
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.
Geany - Programmer's text editor and IDE that is lightweight, customizable, and supports many plugins.
Notepad++ - Simple notepad and source code editor for Windows.
Joplin - Cross-platform note-taking app with end-to-end encrypted note syncing between devices and a handful of other great features.
Turtl - Secure notebook app with an emphasis on collaboration.
EteSync Notes - Self-hostable, encrypted notes app for mobile and web.
LibreOffice - A truly free and complete office suite for everyone.
OnlyOffice - Modern, full-featured Office alternative for collaborators. Excellent compatability with Microsoft Office documents.
CryptPad is a self-hostable, cloud-based office solution that integrates OnlyOffice tools in the web browser, similar to Office365.
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.
Expresso - Webapp for fast neural network analysis of your text style to provide useful feedback metrics to English writers.
LibreTranslate - Machine translation API for text and documents that is self-hostable, can be used offline, and has a free webapp.
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.
Godot - Impressive engine for making professional games in 2D or 3D. Supports multiple languages, including C++, C#, and the developer's own GDScript. It also offers visual scripting tools, VCS integrations (git, etc.), and can deploy to any modern desktop or mobile OS (or even game consoles, with third-party support).
Keychain is a plugin that provides a control re-mapping system for the players of your game.
libGDX - Cross-platform, Java-based game engine behind many popular indie games. Offers a unified API and extensive third-party plugin support.
LÖVE - Framework for building 2D games easily with the Lua scripting engine.
Superpowers - Game engine made with ease of use and real-time collaboration in mind, using TypeScript for game logic. It includes free assets and allows users to publish to multiple platforms, including web browsers.
Spring - An engine specifically built for making custom RTS games with Lua scripting.
Pixelorama - Suite of tools for making sprites, backgrounds, tiles, pixel art animations, etc.
Piskel - Editor for animated sprites and pixel art, works in-browser and as a standalone application.
LibreSprite - Totally free fork from the last GPLv2 commit of the popular open source Aseprite pixelart program. While it's missing some newer features and bugfixes from the current versions of Aseprite, the developers are working on a ground-up rewrite with exciting new features that is worth keeping an eye on.
EZSpriteSheet - Spritesheet packer that can even import from GIFs and WEBPs.