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.
Frame by Frame (Blog)
On Animation
On Animation is a website filled with animation analysis, behind the scenes, podcasts and more. Check it out!
Framepool (FREE+)
Framepool is a footage collection run by filmmakers and has a huge array of stock footage useful as reference material. It can be previewed for free or a higher quality version can be bought.
Animator's Resource Kit 