PhotoGIMP - A patch that makes the UI and keyboard shortcuts much more like Adobe Photoshop, for those that are accustomed to that experience.
Resynthesizer - "Texture synthesis" plugin that functions much like the content-aware filter from Photoshop.
Both Darktable and RawTherapee (see Photo Management below) can also be used as plugins!
Wavelet Decompose - Separates an image into layers based on frequency, with each layer containing a unique details from the original image. This can be helpful for skin retouching or smoothing specific parts of an image.
G'MIC - Offers hundreds of configurable filters for digital image processing.
elsamuko - Small collection of stylish filters and effects.
Beautify - Adds lots of filters with varying texture and color.
ReFocus - Attempts to correct the focus of an image after scanning, scaling, and other manipulations have left the pixels of the image a bit blurry.
ContrastFix - Helps you correct contrast issues in photos using masks.
PurpleFringe - Corrects the "purple fringe" effect that occurs due to chromatic abberation.
Lensfun - Corrects photographic lens errors with a large database of supported lenses.
Hugin - Allows you to "stitch" photos together to make a larger, panoromic image.
BIMP - Allows you apply edits to a whole batch of photos at once.
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.
Tooll 3 - Amazing realtime animation toolkit for motion graphic artists that can be used in audio-reactive VJ content, automating keyframe animation, development of fragment or compute shaders, and much more.
OpenToonz (Morevna Edition) - 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. The Morevna Edition includes a more intuitive UI, brushes from MyPaint, an advanced color selector, and more.
KumoWorks is a cloud rendering tool for OpenToonz animations
enve - Impressive 2D animation and motion graphics software with support for both raster and vector art, sound, and video files.
Synfig Studio - 2D animation software that appears to be no longer actively maintained, though it is feature-rich and provides support for bone rigging, cut-out animation, and traditional frame-by-frame.
Pencil2D - Minimalistic, cross-platform 2D animation software that allows for easy switching between raster and vector workflows.
FreeMoCap - Advanced but usable motion capture framework for everyone!
Infinigen - A powerful tool for procedural generation of infinite, photorealistic worlds with real geometry.
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.
MoonRay - Dreamwork's Monte Carlo Path Tracer renderer used in their animated films.
Pixie - Photorealistic, ray-traced renderer similar to Pixar's proprietary RenderMan.
Audacity - Simple, intuitive multi-track recorder and waveform editor. It has been around since 2000 and remains an exceptionally popular tool for audio recording and editing.
Tenacity - A fork of Audacity from an older version (3.0.2), now taking the project in a new direction.
Wavacity - Another fork of Audacity, but this time ported to the web browser!
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).
Tone - Audio tagger for dumping and editing metadata from multiple audio formats, such as FLAC, MP3, M4B, and more.
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.
MusE - Complete multitrack DAW for Linux that handles both MIDI and audio tracks.
Zrythm - An in-development automated and convenient workstation for music production.
Helio - Remarkably intuitive and concise sequencer with helpful visualization tools.
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.
ossia score - Intermedia sequencer with mixed node-based and timeline workflow.
Ensembles - Digital workstation for arranging music for live performance, not targeted toward studio recording.
Rosegarden - MIDI sequencer and editing tool for composers with classical music notation built-in.
Carla - Non-linear plugin host for running plugins outside the DAW, using Windows VSTs on Linux, and more.
Yabridge - Seamless support for both 64-bit and 32-bit VSTs (VST2 & VST3) on 64-bit Linux hosts.
AudioGridder - Allows users to offload DSP processing to remote computers to save system resources when using intensive plugins.
Element - VST plugin that is itself a modular, node-based plugin host that allows you to chain together plugins to create powerful new sounds.
Calf Studio Gear - Extensive pack of LV2 plugins including instruments and effects in modulation, delay, dynamics, EQ, and more.
Socalabs Plugins - Collection of useful plugins including a great reimplementation of mverb, some basic effects, several synths inspired by retro video game consoles, utility plugins, and more.
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.
IEM Plug-in Suite - Plugin suite comprised of a great multi-band compressor, encoders, decorders, and visualization tools.
Zam-Plugins - Several audio plugins including a really nice dynamic EQ and more, all in LV2 or VST formats.
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 in multiple formats (including classical, jazz chord charts, tableture, vocal arrangements, and more), with a clean UI and support for MIDI keyboard input.
LilyPond - Flexible software for making beautiful, printable sheet music. It is extremely customizable and can be expanded for practically any kind of notation you might need.
Chordly - Easily create simple chord charts with lyrics and transpose chords as needed.
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.
Twine - Tool for developing non-linear, interactive fiction.
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.
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, and includes other features like Kanban and a real-time whiteboard aapplication.
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.
gscan2pdf - Easily make PDFs or DjVus from scanned documents.
Alva - Tool for making "living" prototypes with working code components.
Excalidraw - Generic, intuitive, in-browser sketching tool for quick concepting.
Akira - Offline generic sketching tool, also great for concepting.
Icon Shelf - Icon manager for web developers where all icons are copyable as code.
Meld - Offline tool for comparing and/or merging files and folders on your sytem. Extremely useful for general organization and project management, and can be useful for lots of tasks outside of software development, too.
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.
Armory3D - Powerful 3D game engine that integrates with Blender. Made by the team behind ArmorPaint and ArmorLab.
libGDX - Cross-platform, Java-based game engine behind many popular indie games. Offers a unified API and extensive third-party plugin support.
Panda3D - Engine for real-time 3D games and other applications (visualizations, simulations, etc.).
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.
Fluent Behavior Tree - Behaviour tree library written in C# with a fluent API and good documentation.
SimpleAI - Lightweight behavior tree written in C++ with optional LUA bindings.
Bonsai - Behavior tree written in Rust with nice documentation.
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.