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])
Newgrounds
Action VFX (FREE+)
Action VFX has a collection of free VFX that are useful for adding post effects to your animations. There is also a much larger collection available for purchase.
Animator's Resource Kit 