Texture Packer
Built for use with LibGDX.
Format
In a configuration file (/in/info.txt by default), have a bunch of sprites with the format name_WxH.png, where W and H represent the number of sprites that fit in the image across its width and height, respectively. For example, apple_3x5.png mean a sprite named apple that has 5 rows and 3 columns.
Credits
Demo images are from Jason Perry's excellent Time Fantasy sprite pack (TimeFantasy.net).