Free gamedev resources

Assets

Graphic assets

• OpenGameArt.org — a good old website on which people can post and download their graphic and sound assets.
• Assets by Kenney — probably the most recognizable art style. Public domain license.
• Free assets on itch.io — thousands of artists with different styles. Check the license for each asset pack.
• Freebies of CraftPix.net — high-quality, non-generic assets in various styles and themes.

Free sounds

• Freesound. Check the license for each sound.
• SFX packs by Kenney. Public domain license.
• jsfxr — a web-based tool for procederally generating game sounds.

Free music

• Kevin MacLeod — a full-time composer who has published over 2000 compositions, most of which are made available for others to use using the Creative Commons by Attribution 3.0 license.
• Freesound. Check the license for each music piece.
• FreePD.com. Lots of CC0 tracks.

Learning

Game Design

• Design Oriented Topic Wheel — A huge glossary on gamedev topics made in an interesting interactive way. Make sure to disable DarkReader or other "dark theme" extensions in your browser to actually see the definitions!
• Game Design 101 to 1010 — dozens and hundreds of blog posts on game design.
• Game Maker's Toolkit — high-quality discussion and recommendation on game design by Mark Brown, a known game critic.
• Adam Millard aka The Architect of Games — lots of videos on game design, too!

Game development in general

• Ask Gamedev on YouTube — a group of game industry veterans that make videos on the game development industry and how to make video games.
• Design Doc. Yep.
• Game Developer Conference recordings on YouTube — various talks on every gamedev topic.
• Game Design with Michael — they teach game design theory and talk about the ins and outs of game design so that you are equipped to analyse, discuss and create great games.
• Game Dev Underground — The science, art, and business of making great games.
• Miziziziz makes games, hosts game jams, talks about game development. His videos are short and concise, but useful.
• The game Overanalyser — this channel explores the stories, themes, design and art of gaming.

Pixel art

• Brandon James Greer on YouTube — they make awesome pixelart and tell their workflow in detail.
• Pixel art tutorials on Lospec. Just awesome.

Tools

Animation

• DragonBones — create skeletal animations and import them to ct.js.
• Synfig — Open-source 2D Animation Software.

Graphic editors

• Krita — A graphic editor for artists that also has most features from Adobe Photoshop.
• Gimp — Ye olde monstrosity for raster editing.
• Inkscape — Ye olde vector editor.
• Figma — though it is a prototyping and UI design tool, it has powerful capabilities for vector and is great for illustrations and cartoony game sprites. CoMiGo's go-to graphics tool. Works in a browser and as a desktop app.

Pixel Art focused editors

• Aseprite on GitHub — yes, it is a paid app, but did you know you can compile it by yourself for free, and that it is absolutely legal? If you don't want or can't build it by yourself, check LibreSprite, a fork of Aseprite.
• Piskel — a free online pixel art editor. Supports both animations and layers.
• Lospec Pixel Editor — another online pixel art editor. Supports layers.
• Lospec Procedural Pixel Art Generator — an interesting app that creates variations of sprites based on randomized pixel placement.