I would like to share a tool I have developped when working on Plee the Bear and Andy's Super Great Park. This is a sprite packing software that works on Gimp images and produces PNG sprite sheets.
The software is Pack My Sprites. It is written in C++ and relies upon Gimp and Xcfinfo to build the sprite sheets. The source code is available under the terms of the GNU GPL version 3. Only the source code is available now, there is no binaries yet.
The website {l Wrote}: Pack My Sprites is a program that generates sprite sheets from a collection of images made with Gimp. The process does not requires the user to produce any intermediate image files. Thus, all you have to do is drawing the image and listing which layers to use for each sprite, then the program will generate the sprite sheet from it. Pack My Sprites makes the modifications on the source images costless for the project.