Reference Images, Concept Art and 3D Models

Models Resource is an excellent website for finding 3D models from video games. You can find models from games like final fantasy VIII, Zelda, Mario, Crossy Road, Sonic, Super Smash Bros., Digimon, Pokemon, One Piece and many more. They also have games from a wide range of old and new platforms including: Nintendo 64, Game Cube, Wii, Wii U, 3DS, Playstation 1, Playstation 2, Playstation 3, PSP, Xbox 360, and PC. Check it out! It is a great place to learn what professionally made models for video games look like. Use the models to help you design and improve your own models.

Living Lines Library
This is the blog of an animator who has worked in 2D and 3D animation. Here you can find concept art,  from movies such as Akira, Coraline, Hotel Transylvania, My Neighbor Totoro and many many more! He also has a lot of free books, and documentaries about animation too! It is quite difficult to find this type of stuff online, so make use of it!

If there are any other websites that you use, please let me know and I will add them to this list.