Grease Pencil is a feature inside Blender that allows you to animate in 2D while taking advantage of a full 3D suite of features. Definitely one of the best pieces of software that blends the two together so seamlessly (and free!)
Grease Pencil
Krita
Krita is a free app for digital painting but also supports 2D animation. It is made by artists that want to see affordable art tools for everyone.
Live2D
Live2D has a bunch of software that allows 2D artists to create animations from their artwork. Available in both FREE and PRO versions.
The Fighters Generation
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!
Iamag.co
Iamag.co is a huge collection of references for walk cycles. The collection contains illustrations, sketches, model sheets and tutorials…
OpenToonz (FREE)
OpenToonz is a free open-source software for producing 2D animation and is known for being used by Studio Ghibli
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.
Howard Wimshurst Tutorials
Howard Wimshurst has a youtube channel filled with some great animation techniques that apply across both 2D and 3D disciplines.
Pencil 2D (FREE)
Pencil 2D – An animation software for vector and bitmap graphics. It is free, multi-platform and open source
Animator's Resource Kit 