Frame by Frame is a blog that displays sequential images from various animated films for study and appreciation. It also shows the gif to supply reference for timing.
Animators Resource is a similar website to this one but focuses more on specific posts and videos rather than linking to other websites.
On Animation is a website filled with animation analysis, behind the scenes, podcasts and more. Check it out!
Spungella is a blog run by Jean-Denis Haas that is constantly updated with inspiring animations, short films, progression reels and more. A great resource for inspiration!