One of the more prominent ones is ToonBoom Animate
http://www.toonboom.com/products/animate/features.php
It costs but it's there...you can use joints, keyframing, substituting drawings, etc.
Also, knowing ToonBoom, there SHOULD be a Personal Learning Edition, which is basically an unlimited trial.
I don't think it's totally meant for animations the way we're doing it, but it should allow for it.
There was another one I saw a while back but have since not been able to find it. It stuck out because it had a really cool system of drawing bones and joints out on the 2D image for animation. I'm pretty sure it was free but I could be mistaken.
EDIT:
I still say I can't draw...
Turner Sketch
Unless you want crayon graphics, I'm not the one for this.
xD