Newgrounds is a popular website for posting animations, games and audio and has been around for over 20 years. Founded by Tom Fulp (also founder of The Behemoth [game developer])
Sketchfab allows you to upload full 3D scenes for others to view directly in their browser. They are the world’s largest platform to publish, share and discover 3D content online and in VR.
Artstation – Mostly dominated by artists but still a good place for animators to upload their work
Vimeo – A great website for submitting animations, creating a profile and connecting with other animators