SloMo Ref is a free collection of slow motion animal reference videos on Youtube
Endless Reference is a Youtube channel that focuses on animation reference. Lots of great human locomotion videos from different angles
Bodies in Motion is an ongoing photographic project that aims to document the power, beauty and possibility of the human body in motion. It is freely accessible to all visitors at preview resolution or you can subscribe for higher quality images.
MakeHuman is a free open-source software that allows you to create and pose 3D human characters. Useful for pose reference.
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.
The Fighters Generation is a world renowned character database… featuring just about every fighting game character in existence. It boasts an unparalleled collection of the best quality artwork, pictures & animations!
Cartoon Network D.O.C is an amazing site for viewing storyboards, model sheets, backgrounds and more from cartoon network shows.
Iamag.co is a huge collection of references for walk cycles. The collection contains illustrations, sketches, model sheets and tutorials…
Brendan Body has a great post on his website which displays lots of great reference and observations on birds and flight.