Krita is a #free💬 and #opensource💬 tool for creating digital art, including animation! And in Krita 5 they've introduced motion tweening! So now you can use motion tweens by going to "Settings->Dockers->Animation Curves" and you can move a whole "group" of layers as one thing.
OK, it's basically like flash. Like flash, you can just use it for animation, but you can also make it jump from frame to frame and it interprets Javascript code so it can get quite advanced.
Basically, you can use it for frame-by-frame animation, but you can also utilize "tweens" to make some things easier, and you can even make a game with it just like flash.
One thing I really like is that it has the "tween" just like flash. This means, you take an object, make a keyframe, make a second keyframe, move the object, and Wick will calculate all the in-between frames for you. Very useful!!!
Something I don't like is there doesn't seem to be a "skew" tool. So, you can't have the same bounce and stretch that you could have from SynFig or OpenToons. At least, as far as I can tell from my limited experience. Still, a very nice tool. Easy! Probably a good beginner program for kids, too!