Most developers simply create their own tools; there is no standard format that I know of.
Part of the reason for that is it's a relatively straight-forward task once you have written the code to display such a keyframe. It's a trivial step from that to add some basic functionality to move and rotate each bone, and save the set of transforms as a keyframe. From there, you need only add some controls for the timeline, and you have yourself an editor!
That said, I was able to find this editor, though you'll have to write the loading routines yourself: http://www.allegro.cc/forums/thread/590509
It's probably not what you're looking for but JME has support for skeletal animation: http://www.jmonkeyengine.com/wiki/doku.php/animation.