Game Engines
Freesound
Free collaborative sound library offerer with different CC licenses
Kenney
Free (CC0 1.0 Universal licensed) 2D, 3D, Audio, and UI game assets
OpenGameArt
OpenSource Game Assets like music, sounds, sprites, and gifs
Game Icons
Free styleable SVG/PNG icons provided under a CC-BY license
GameDevMarket
Free/Paid assets like 2D, 3D, Audio, GUI
Poly Pizza
Free low poly 3D assets
3Dassets.one
Over 8,000 free/paid 3D models, and PBR materials for making textures
Gamefresco.com
Discover, collect, and share free game assets from game artists everywhere
itch.io
Free/Paid assets like sprites, tile sets, and character packs
LoSpec
Online tools for creating pixel art and other restrictive digital art, lots of tutorials/pallet list available to choose from for your games
ArtStation
MarketPlace for Free/Paid 2D, 3D assets & audios, icons, tile sets, game kits. Also, It can be used for showcasing your art portfolio
Poliigon
Free and paid textures (with variable resolution), models, HDRIs, and brushes. Offers free plugins to export to software like Blender