UPDATE: The app is now RIVE.app
If you are looking for Vector / 2D animation tools, Nima and Flare from 2Dimensions is a free 2D animation tool with Bone rigging. Currently it comes in 2 flavors, Flare is for SVG Vectors and Nima is for 2D PSD layer type animations.
I am trying to experiment rigging a small 2D Pixel Character. For Nima you need to prepare your assest in PSD or similar and import each character part you want to animate. Flare instead allows you to draw in the Canvas.
A timelapse video of animating in Flare by 2Dimensions
Once you are done with your animation, Nima can help to produce Gif animations for Flare it can export to various game engines like Unity or Mobile Flutter a Google API for making Apps.
I am surprised they have you able to make projects for free with a paid option to keep your files private. Its a very good deal. They are trying to support Open Design, which is to share your process and how you got to the final image.
There are a bunch of quick start tutorials on thier blog