Simply the largest and most robust game asset site you can find. As a developer, you must have heard of the Unity game engine. It has been used to create games such as Assassins Creed, Deus Ex, Hearthstone, Ori and the Blind Forest, Cuphead, etc.
Apart from game development, the Unity engine is also used for architectural visualization through a virtual reality environment. There is a massive community build around the Unity engine, comprised of indie devs, AAA giants, college students, and 3D artists.
You have access to more than 1000 free 3D models, and these are on average better designed compared to 3D assets you’ll find on nonprofit sites.
Just download a free copy of the Unity engine, access the store, download whatever free assets you want, and they will be automatically imported into Unity. You can save these assets in a project folder and use them in any 3D game engine.