GIMP - One of the most popular Photoshop alternatives with an impressive feature set and versatility.
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.
PhotoGIMP - A patch for GIMP that makes it 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 for GIMP!
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.
Darktable - Virtual lightroom and darktable for photographers, allows for processing of raw image files.
RawTherapee - Like Darktable with a steeper learning curve, but with excellent HDR and tonemapping capability.
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 (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
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.
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.
Audacium - Another Audacity fork. Though no longer actively maintained, 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.
Zrythm - 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.
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.
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.
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.
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.
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.
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.