There is a free animation package called Blender. If you are doing any form of animation it is worth a look.
In the time it takes me to paint a single frame in a raster editor, I can have a complete walk cycle complete in Blender.
The UI is made for professionals who use hotkeys. Make sure you learn the hotkeys, rather than using the UI, and you will be a pro in under a month.
In 2 months of daily tutorial reading, and you will be up there animating with the best of them.
Blender is also handy for creating scenes, and tiles sets.
For the price it is insane not to at least try it.
If you are not much into the modeling side, you can use utilitys like Make human to rapidly make humans, zombies and mutants. They even incluse a modeled bone skeleton!
The rigs it exports are all ready named for BVH stop motion animation. You can make a human, fully clothed, and have them running,climbing, and jumping around in less than a half hour per character.
Proper cycles will be needed for walking and running and such, but those are all downloadable animations.
I already had training for modeling and rendering architectural visualizations in Light-wave when I first started, but that only helped me out with the basics of modeling animated things.
I learned the basic's of IK rigging and animation with Blender in an afternoon. After about a week I had all the constraints, and a few basic animation theories down. After about a month my characters started really becoming life-like.
The Blender forum has some great tutorials on how to make your creatures lifelike, and believable.