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OK, so you may have seen my collection of Libre Game Assets, but why do the tools also need to be Libre/Open-Source Software if you're not going to redistribute them anyway? Isn't freeware enough?

Here are some good reasons:

1. You're guaranteed to be able to keep a backup, in case the creator goes away or turns evil and open-source licensing helps to give you leverage against techno-feudalism.
2. If it's in a programming language you or a friend know, you can customize it to meet your needs without having to beg its creator.
3. If the creator loses interest, someone else can take over, unlike some of the freeware Windows 9x utilities I remember fondly from my childhood.

This is a list of tools (and demo apps you might want to incorporate source from into your games) that you can collect on a thumbdrive, take with yourself to a desert island, and give to a friend, all without any worries.

And, like with assets, if you like it, try to find a way to give something back. (If you can't afford to donate, maybe fix a bug or help to get them some free publicity.)

NOTE: As demonstrated by games like Ocean's Heart, you can publish commercial games made on GPLed engines, because the art assets don't count as derived works of the engine software. It's similar to how just because GOG.com sells classic adventure games (the AAA titles of their day) running in ScummVM engine (a replacement engine that's open-source) doesn't mean the game resource bundles are suddenly free to redistribute.

See Also:

• Azgaar's Fantasy Map Generator (Open-source in-browser map generator. MIT license.)
• BMFont (Generate bitmap fonts from TrueType fonts. Zlib license.)
• Effekseer (Particle effect creator with gallery of CC0-licensed samples and Godot 3 and Godot 4 integrations. MIT license.)
• FontForge (Create or edit both vector fonts like TTF/OTF/etc. and bitmap fonts like the FON/FNT/etc. files that some retro font loaders expect. Be aware that the SIL Open Font License requires you to give modified versions of fonts a new name. GPLv3 license.)
• Open Match (Open-source matchmaking server. Apache 2.0 license.)
• Papagayo (Helper for syncing lip animation to recordings, Blender-compatible. GPLv3 license.)
• SFXR (MIT), Bfxr (Apache 2.0), and GodotSfxr (MIT, GitHub) (Drag sliders or mutate presets to build retro sound effects)
• SHADERed (IDE for writing shaders. MIT license.)
• My List of Installer Creators (Useful for DRM-free Windows downloads and retro-authentic games. If you just want the easiest-to-learn option for modern Windows, use Inno Setup and see my example .iss file. Inno Setup is under a custom permissive license and GOG.com installers for Windows use a customized build of Inno Setup.)

Game Engines:

• Anura (The platformer engine, tools, and documentation written for Frogatto & Friends. Has a level editor that you can drop into at any moment with Ctrl+E. Engine source under Zlib license.)
• GDevelop (The closest open-source thing to Game Maker Studio. Uses JavaScript instead of GML when you want to move beyond visual scripting. Funded by optional subscriptions for cloud services but the engine, editor, and core extensions are under the MIT license. Has an in-browser version with guided lessons.)
• Godot (The closest open-source thing to Unity and already better in some ways. Also in the collection proper, but it's duplicated here so people don't miss it. See also: Unlearn Unity: Godot for the Solo Developer, both for how to approach Godot and why its design philosophy may help your motivation. Fully cross-platform. MIT license.)
• Solarus (GPL3-licensed engine and editor GUI for top-down 2D action RPGs. Has an MIT-licensed starter pack. See the license guide for more. Used to create the game Ocean's Heart, which is on GOG.com.)
• Spring Engine (Open-source (GPLv2) 3D RTS engine with its own lobby system.)
• Stride Engine (Another open-source 3D engine with a Unity/Unreal-like integrated editor and workflow. Editor only runs on Windows. As of 2023-10-06, Linux output support is experimental and macOS output support is missing. MIT license.)
• Adventure Game Studio (Open-source engine and editor for point-and click adventure games. Used successfully in various commercial games good enough to have made it onto GOG.com such as Quest for Infamy. Artistic License 2.0.)
• DIV Games Studio (Original) (A game engine and IDE, originally only for MS-DOS, intending to be a DOS-era occupier of the Unity/Godot-style niche. Now open-source under the GPLv3 and ported to many platforms.)
• ...see also entries in the collection below, such as GB Studio and Ren'Py. (Both MIT licensed.)

Game Frameworks and 3D Engines:

• Allegro (If you're writing a DOS game engine, Allegro is similar to SDL and 4.2 is the last version to support DOS but also more modern OSes. However, Allegro 4.x mouse support can have performance issues on Linux. Allegro 4 is under a 0BSD-equivalent "giftware" license.)
• MonoGame and FNA (If you learned XNA 4, FNA aims to be a 100%-compatible open-source, cross-platform version and has been used to port many games, while MonoGame aims to be an unofficial successor to XNA 4. MS-PL Licensed.)
• HaxeFlixel (If you learned Flash, this is a Flixel port to an open-source, compiled variant of ActionScript. Defender's Quest was rewritten in this. MIT License.)
• LÖVE (Framework for writing 2D games in Lua. Zlib/libpng license.)
• LoveDOS (A game-making framework for DOS based on a subset of LÖVE. MIT license.)
• O3DE ("Open 3D Engine". A spin-off of Amazon's Lumberyard, itself derived from Crytek's CryEngine. Apache 2.0 license.)
• OGRE (Portable 3D engine written in C++ with Java, C#, and Python bindings. It was used to make multiple commercial games. MIT License.)
• SDL 2.x (If your target OS supports it and you're writing an engine, use SDL 2 or a binding to it like rust-sdl2. This is what Valve recommends and they integrate controller auto-configuration for it into Steam. Zlib license.)

Interactive Fiction Engines:

• Twine (The most novice-friendly Interactive Fiction engine. GPLv3 license.) 
• Inform 7 (A nice middle-ground. Not visual programming, but the IDE is half-way there with a narrative-like syntax, an auto-mapper, and various other more-than-text views. Artistic License 2.0 as of 2022.)
• TADS (The most advanced, but the least hand-holding.)
  NOTE: The TADS authoring tools are under a "freeware source license" that prohibits commercial redistribution. However, multiple third-party runtimes for compiled TADS games exist under GPL licenses, such as Spatterlight and Zoom for macOS and QTads and Gargoyle for Windows and Linux/UNIX.
• ...see also: Trizbort (MIT)

3D Modelling:

• Blender (THE open-source 3D modeller. Can also be a non-linear video editor or a digital animation tool. [1] [2] GPLv2 licensed.)
• Wings 3D (Subdivision modeller. Good for producing models for Blender. I don't recognize the license, but Debian is OK with packaging it.)
• MakeHuman (Standalone humanoid model generator. AGPLed, but assets are CC0 and project is of the opinion that no restrictions will be imposed on what you generate.)
• MB-Lab (Blender plugin for humanoid model generation. CAUTION: Project is of the opinion that generated 3D Models will be AGPLed because the mesh database is AGPLed. Probably best to use its sibling, MakeHuman, instead.)
• Natron (Blender-compatible VFX compositing intended to compete with Adobe After Effects.) (GPLv2)

3D Plant Generation:

• ngPlant (GPLv2)
• Arbaro (GPLv2)
• An Ivy Generator (Original Site) (GPLv2)
• Godot Tree Generator Plugin (MIT)

Illustration and Digital Painting Tools:

• Animator Pro (Open-sourced Autodesk Animator for DOS. The native FLI/FLC format can be loaded and played by game frameworks like Allegro 4.2. 3-Clause BSD license.)
• The GIMP (The closest open-source competitor to Adobe Photoshop. While neither are explicitly intended for pixel-art, some people use them quite successfully. GPLv3 license.)
• GrafX2 (Open-source, cross-platform Deluxe Paint knock-off. GPLv2. GitLab)
• Inkscape (SVG Editor with PNG export. GPLv2 license.)
• Krita (Raster-based, aims to compete with Corel Painter and SAI. GPLv3 license.)
  
• LibreSprite (A fork of the last GPL-licensed version of Aseprite. GPLv2 license.)
• MyPaint (Raster-based digital painting tool like Krita. GPLv2 license.)
• Pencil2D (2D Animation, such as for cutscenes. See also Synfig Studio and Blender. GPLv2 license.)
• Piskel (Free and open-source browser-based sprite editor with downloadable versions available. Apache 2.0 license.)
  
• Pixelorama (An up-and-coming free alternative to Aseprite. Also in the collection proper, but it's duplicated here so people don't miss it. MIT license.)
• rx (Minimalist, modal animated sprite editor. GPLv3 license.)
• PikoPixel (Simple open-source pixel art editor for Mac, Linux, and BSD. AGPLv3 license.)
• Synfig Studio (2D Vector Animation, such as for cutscenes. Has tweening and bones. See also Blender. GPLv3 license.)

Music Production Tools:

• Ardour (Big fancy DAW. GPLv2 license.)
• Audacity (Basic Multi-Track Sound Recorder/Editor... licensed under the GPLv3 but it's been bought by a company that's starting to add analytics and other iffy stuff to builds, so a fork like Tenacity (GPLv2) would probably be a better choice.)
• axiom (Modular synthesizer, standalone or VST2. MIT license.)
• FamiTracker (MOD tracker-style tool for producing NES chiptunes. See also FamiStudio (MIT) in the list below for a DAW-style alternative that's easier to use but lacks some of the more niche features. GPLv2 or GPLv3 depending on which fork you use.)
• klystrack (Cross-platform chiptune tracker, MOD/FT2/SID import, WAV export. MIT license.)
• LMMS (Resembles FL Studio. Has nice synth plugins including Gameboy, NES, SID, and sfxr as well as SoundFont support. GPLv2 license.)
• MilkyTracker (Cross-platform MOD/XM tracker, GPLv3 license.)
• OpenMPT (Windows IT/XM/MOD/S3M tracker, intended to work well in Wine. 3-clause BSD license since v1.17.02.53)
• Stargate (Cross-platform DAW aiming to be a powerful, simple competitor to Ardour and the like. GPLv3 license.)

VST/AU/LV2/etc. Synthesizer Plugins:

• ADLplug (GitHub) (BSLv1)
• amsynth (GitHub) (Linux/macOS only. GPLv2 license.)
• dexed (GitHub) (GPLv3)
• Dragonfly Reverb (GitHub) (GPLv3)
• Helm (GitHub) (GPLv3)
• Ninjas 2 (GitHub) (GPLv3)
• OB-Xd (GitHub) (GPLv3)
• Odin 2 (GitHub) (GPLv3)
• Surge (GitHub) (GPLv3)
• VCV Rack (GitHub) (GPLv3)

Godot Addons:

• Smoothing Addon (Timestep interpolation for Godot) (MIT)
• Integer Resolution Handler (Alternative stretch handler for pixel art games. Godot 4 pending as of 2023-10-05) (MIT)
• Cyclops (Block out 3D levels right in Godot 4) (MIT)
• Dialogic (Dialog tree plugin for Godot 3.5 and 4) (MIT)
• EXP Godot Dialog System (Godot 4 pending as of 2023-10-05.) (MIT)
• 1$ gesture recogniser and recorder (It's free. The algorithm is named "1$") (MIT)
• godot-fancy-scene-changes (Scene transitions. Godot 4 pending as of 2023-10-05) (MIT)
• godot_heightmap_plugin (Heightmap-based 3D terrain for Godot 3 and 4) (MIT)
• godot-jolt (Drop-in replacement for most of Godot 4's physics engine with better performance, as well as some optional extra features. See the README for details) (MIT)
• Godot Pixelorama Importer (Godot 4) (MIT)
• Godot_RegularPolygon2D (CAUTION: Unlicensed)
• godot-rust (Write compiled extensions for Godot 3 and 4 in Rust. Great for stuff that needs as much CPU as you can throw at it, like complex procedural level generation. Also check out Rayon) (MIT for Godot 3 and MPL 2.0 for Godot 4)
• Godot Mixing Desk (Procedural/adaptive sound and music for Godot 3) (MIT)
• Godot Meta Player (Port of Godot Mixing Desk for Godot 4) (MIT)
• Godot LOD (Adaptive LOD for 3D in Godot 3.x) (MIT)
• Qodot and Qodot 4 (import Quake .map files as 3D geometry so Quake level editors can be used as a novice-friendly way to do 3D modelling. Has extra integration with TrenchBroom, which is GPLv3.) (MIT)
• RNGTools (MIT)
• SmartShape2D (An alternative approach to 2D terrain for Godot 3 and 4) (MIT)
• Waterways (Easily make flowing rivers for 3D scenes. Godot 4 pending as of 2023-10-05) (MIT)
• Awesome Godot (A categorized list of addons and tools for the Godot engine)
• A bunch of addons in the list below