Line of Action is a really useful site for learning about the human form, anatomy, speed drawing and more. Useful for both 3D and traditional animators!
Line of Action
Wowhead
Wowhead is a site that allows you to view character animations from World of Warcraft. Another great source for game animation.
LOLKing
LOLking is a website that can be used to view League of Legends characters and their animation sets. Great as a source of game animation reference!
Zweifuss
Zweifuss is a website that displays the sprite animations for Street Fighter 3 – Third Strike. Great for reference material and for just admiring the lovely sprite animations.
Eadweard Muybridge
Eadweard Muybridge was known for his photography of humans and animals in motion, a lot of which can be found on google and is a great resource for reference material
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.
GettyImages (FREE+)
GettyImages has a massive selection of stock footage and is an amazing resource for reference material. Previews can be watched for free or you can pay for a higher quality version.
Pinterest – Animation
Pinterest is a great place for animation inspiration and reference material. Whether you search for animation, dynamic poses or facial expressions you’ll always find something useful.
Animator's Resource Kit 