OpenToonz is one of the animation apps that I use. It is open source, free, and very powerful. It is a production-level tool used by various Japanese animation studios. An older version was in use by Studio Ghibli, to scan and colour all hand-drawn frames, and add effects, etcetera.
I also use Krita for more painterly animations. Also free and open source.
And Clipstudio EX, which is excellent for line work and animation. It exports directly to OpenToonz to compile all animation scenes into a full render.
Lastly, Blender: for 3d animation, and it's pretty cool for cut-out 2d animation as well. And it supports frame-by-frame hand-drawn animation now too, but I haven't used it for that yet (tested the waters, and it is pretty cool!).
I used to do cut-out animation in Moho Pro, but switched to Blender a few years ago. And the DUIK team is developing tools for Blender now, which is exciting! (google DUIK for After Effects to understand my excitement!).
Anyway, OpenToonz & Krita are probably all you need, plus Blender.
Using an older version of Flash CS6 & the free Flanimation plugin is pretty good too.
Or grab a pencil and sheets of paper, animate on paper, and scan your drawings in OpenToonz. OpenToonz will vectorize your drawings for easy colouring.
