itch.io

See Also: Libre Game Dev Tools 

Game assets which should be usable for DFSG-compatible open-source games.

(i.e. assets which were either released into the public domain or were offered under a license compatible with the terms of the Debian Free Software Guidelines at the time I downloaded them and, thus, even if the creator changes their mind later, access remains under the old terms via anyone who had already acquired them and wants to pass them on.)

Why?

There's a lot of legal ambiguity in casual "do what you want" license terms and "no redistribution" terms mean that, if a site goes down, all those resources are lost or legally poisoned forever. (Given how copyright keeps getting extended.)

The requirements of the Debian Free Software Guidelines were crafted based on specific concerns about how a bad actor could make people miserable or cripple their competition or how a disaster could do likewise.

For example, Debian checks for things like license non-revokability via a thought experiment they call  the "Tentacles of Evil" test because it's intended to protect against an evil corporation or government coercing or buying out the creator and then making life miserable for people using what they created.

At the same time, the "Desert Island" test disallows "no redistribution" restrictions, because we don't want your game assets to die with you or the site you posted them to, and "no reselling" is also disallowed because all three big definitions of free/libre/open-source software (FSF, OSI, Debian) agree that "restrictions on fields of endeavour" aren't OK.

(Abusive behaviour is instead mitigated by requiring that you give credit to the creator and, if it's a concern, by using a license like the CC-BY-SA or GNU GPL which requires modified versions to be under the same terms. Thus, ensuring a free supply of the specific icon/background/etc. to compete with the bad-faith actors trying to sell it while allowing good-faith actors to do things like selling Wikipedia on DVD to schools in developing countries with poor Internet access for some token "cost of labour and materials" fee.)

Also, personally, I see inclusion in curated Linux package repositories like the Debian package repository as a great thing for a free game to aspire to. If your entire game isn't under DFSG-compatible licenses, you'll have an uphill battle on your hands to get it into the default package repositories for Linux distributions, since they reserve their "non-free" sections for things they carry under protest but can't avoid, like "gratis but not libre" firmware blobs needed to make your hardware work.

Examples of compatible licenses include the CC-BY and CC-BY-SA but not the ND (no derivatives) or NC (non-commercial) variants. Releasing things into the public domain is also fine, but the CC0 public domain dedication is preferred because it includes a fallback to an equivalent license in jurisdictions like Germany where you're not allowed to prematurely give up your copyright and you have to cleverly work around "you can't pre-emptively give up rights that get written into law later on".

NOTE: The Free Software Foundation has a good explanation of why informal "do whatever you want" licenses aren't safe for inclusion in this list. If it's not under a proper, formal license at the time I look at it, it's not getting on this list.

Can I Use These In A Commercial/Closed-Source Game?

Yes. Many popular indie games have used libre assets.

If the assets are in the public domain (eg. CC0), then you can do whatever the heck you want, but don't be a jerk. Give credit anyway. (Covering your behind this way will also improve the chances that unbalanced people will go after the original asset creator instead of you.)

If the assets are under a "permissive" license like the CC-BY or the MIT license, all you need to do is give credit to the author in the form they specify. (eg. If they want a link to their site, you have to include a link or URL in your credits rather than just their name.)

If the assets are under a "copyleft" license like the CC-BY-SA (Don't worry. Almost none in this list are.), then you want to make sure that your game is seen as an "aggregation" of the libre bits and the bits you don't want people to be able to copy without paying, rather than a "derived work".

(A "copyleft" or "hereditary" license is a license where, if you modify something, the modified form has to be shared under the same license but, suppose you want to put a bunch of resources in a Zip file for a friend. The law doesn't suddenly say that everything else in the Zip file is "derived from" the copy of GIMP that you added.)

For example, if you want to use a libre icon set that's under a copyleft license:

1. Clearly document where everything came from, as with a permissive license. The #1 thing all libre licenses care about, permissive or copyleft, is making sure that it's clear who made what. (Often, the license will require that you retain the block of copyright text in the form the author provided and it's also a good idea to include a copy of the license itself.)
2. Make sure anyone who buys your game can easily extract the copyleft-licensed assets. If you don't have them sitting loose in a folder and you're not using Zip as your packfile format, the easiest way is to just have a second copy of them in a folder as part of your download.
3. Comply with any extra terms of the license. The GPL was designed for source code, so, in addition to being copyleft, it requires that you share the "preferred form for editing". For example, if your PNGs were generated from SVG files, then it's the SVG files that you need to make available to buyers who want them.
4. Design your game to be as easily modded as possible. A media player isn't considered a "derived work" of the audio/video files it plays, and your game won't be a derived work of the art assets it uses if anyone who buys it can look at the format and X-by-Y pixel dimensions of your sprite sheet, drop in a solid-color image as a replacement, and still have the game technically functioning. (eg. to replace an immersion-breaking Twitter or Facebook button on the menu screen with a fully transparent image)

...and, no matter what license it's under, if you like what you got, try to give something back. (Whether it be a donation, releasing some libre assets of your own to pay it forward, or even just spending more than the base minimum effort helping to get the creator some publicity.)

See Also:

• OpenGameArt.org (2D, 3D, Fonts, Concept, Music, SFX, tutorials, etc.)
• Game-Icons.net (Vector icons)
• GodotShaders.com (Ready-to-use shader code for Godot games)
• Kenney.nl (Public domain (CC0) 2D, 3D, Audio, and UI assets)
• Lospec (Pixel art palettes, tutorials, and in-browser editor)
• SpriteLib (A CPL-licensed pack of sprites)
• Subtle Patterns (Textures intended for website backgrounds, also on GitHub)
• Artbreeder (Easily create CC0-licensed art by blending images and steering an A.I. toward what you want)

3D Models:

• MakeHuman (Standalone humanoid model generator)
• MB-Lab (Blender plugin for humanoid model generation)

Textures:

• CC0 Textures
• Texture Haven
• Outworldz.com - Free Creative Commons seamless textures
  
• The Materials section on the Material Maker website

Sound Effects:

• FreeSound.org
• SFXR and Bfxr (Drag sliders or mutate presets to build retro sound effects)

Music:

• ccMixter for Video Games
• FreeMusicArchive.org
• FreePats (Instrument samples for music creation)
• FreePD
• Incompetech by Kevin MacLeod
• Musopen (Public domain and CC-licensed recordings and sheet music)
• Mutopia (Libre sheet music as PDFs you can perform or MIDI files you can feed to something like LMMS, Ardour, or Rosegarden to make your own cover.)
• SampleSwap (Instrument samples and audio loops for music creation)

Fonts:

• Open Font Library (Vector fonts. See also BMFont for converting to bitmap)
• matson48-fonts (Libre bitmap font with a full set of codepage 437 glyphs)
• CG Pixel 3x5 and 4x5, Proportional and Mono (Smallest you can get, CC0)
• 3 Tips for Super Legible Pixel Fonts (Because making your own is so viable)
• Godot-Fontpack (A variety pack of libre-licensed vector fonts suitable for novices, game jams, and anyone else who needs to get up and going quickly.)

Other:

• OpenClipart.org (Public domain vector art. See also FreeSVG.org and FreeSVGClipart.com for other sites which forked when they were down for a while.)
• Wikimedia Commons (Images that could be used for art references or traced over to produce something that fits a game's aesthetic)
• The Internet Archive, The British Library on Flickr, and Project Gutenberg (Content old enough to have fallen into the public domain which could be incorporated into a game. For example, a torn out page of Frankenstein or Dracula being a clue in a horror game or a 19th century illustration repurposed for a gothic-styled work of interactive fiction.)
• Public Domain Review (A blog that trawls through public domain stuff to bring attention to really interesting bits.)
• Procedural Content Generation Wiki (Useful reference materials. See also RogueBasin.)