I funded this awhile ago, and it was very relevant for a project I was working on, but if you guys are interested- the KickStarter has it discounted quite a bit:
http://www.kickstarter.com/projects/esotericsoftware/spine
I'm using it instead of flash for animating characters for my game, and it does a good job, as well as being lightweight.
They also have runtimes for libGDX, cocos2d, and since they made their goals, AS3, C#, javascript, ect.
Made by one of the two guys who are the main contributers of LibGDX.
Definitely more than worth it if you intend to ever use 2d skeletal animation in a project.
I spent a couple months writing a AIR program to convert flash XML files into easily read XML/JSON files, then loading them into my game, and it is still quite a ways off their app (That also includes the program to actually animate).
You can also buy it not off their website, but for a bit extra outside of the KickStarter(if you can't stand waiting 2 more days)
http://esotericsoftware.com/uncategorized/kickstarter-stretch-goals/
Spine - 2D skeletal Animation software + runtimes KickStarter- discounted for 1 more days!
Spine-