I wouldn't know of any programs that have good tweening, mostly because I never use the tweening in 2d animation, but as far as frame by frame stuff goes, I'd recommend Krita if you want to use layers in your drawings. Sketchbook also has a simple animation tool that also works, but it doesn't allow you to use layers when using the sketchbook feature.

Krita is pretty good for frame by frame (and free!), no tweening though.
Blender is actually pretty good at 2d stuff now too, but it takes more learning since its a bit different than drawing programs. It has tweening, and a shit ton of other stuff.

CSP EX is great, a lil different than flash but has no tweening or stuff (which i never used anyways). Its a one time purchase and I think its worth it since CSP is a such great program.

I'll throw Opentoonz in here. It has both frame by frame and tweening. It may seem a bit daunting to learn how the program works, but the features and capabilities are there. If you have some question feel free to ask.