If you are interested in trying to use computer software for traditional 2d animation and tweening, it is ESSENTIAL that you have the option to use PIVOTS or BONES. These help to prevent elements and bodyparts drifting apart during movement. If you are more interested in traditional frame-by-frame, cel-style animation, then neither pivots nor bones will be of much interest to you. (Free programs such as PENCIL or EASYTOON work in that way).

Software featured in this simple demo are:

Anime Studio Pro (commercial software),
Creatoon (freeware),
Express Animator (free trial shareware, see warning),
Pivot (freeware),
Skeleton (freeware).

There is also a comparative demo featuring Artoonix (free with restricted functions; register for full access) which is a sort of ‘cross-over’ program which uses frame-by-frame animation AND tweening (when you register it) BUT does not as yet offer anything more than a single, movable pivot. WARNING — Although ‘Express Animator’ still SEEMS to be under development, there are frequent problems registering with the forum (or even accessing the forum) and the developer(s) seem to be exceptionally poor at keeping users up to date with progress. Personally, I would suggest that you think twice about registering the software until and unless it looks like it truly is going to be completed. I suspect that it could be a hobby project. It has the feel of something that could have started life as a university-degree assignment. I have encountered this phenomena before and in both cases, the projects foundered when the developers found full-time employment. I hope I’m wrong - but if you decide to check out this program, at least begin by following the homepage link which puts you in contact with the developer (Paul) and see if he will assure you that the project is going to be finished.