Unity Asset Store - Unity's asset marketplace for game makers.
Kenney Assets - We've created over 40,000 images, audio files and 3D models for you to use in your projects. Thanks to our public domain license you're even allowed to use them in commercial projects!
GameDev Market - GameDev Market is a marketplace for high quality, affordable 2D, 3D, GUI & Audio game assets, handcrafted by talented creators around the world.
OpenGameArt.org - A site dedicated to sharing artwork & other assets for game development.
Scirra Store - Marketplace for royalty free game assets and tooling.
Freesound - The Freesound Project is a collaborative database of Creative Commons licensed sounds.