I have decided to open source my 2D spritesheet and animation tool. It supports automatic sprite selection, combining images, sprite grouping, and multi-sprite animation with rotation.
It is written in Java, the github repo is here: https://github.com/darkFunction/darkFunction-Editor
Website for the project is: http://darkfunction.com/editor
Everybody loves screenshots so here you are:
Thanks and I hope you find it useful.