I have been playing with a really nice game framework for iPhone called cocos2d in the last few days. One of the new features in 0.7.1 of cocos2d is the introduction of Sprite Sheet. It took me some time to get the first sprite sheet up and running. I researched around and there doesn't seem to be a tool out there for creating sprite sheet (except for one tool from Microsoft XNA but that's for windows only). So I went ahead and whipped one up in AIR yesterday. If you are creating sprite sheets for your games, you might find this handy.The app assumes that you already have the individual PNGs prepared and suffixed with digits (e.g. sprite1.png, sprite2.png, sprite3.png, ...).
Download Atlas Sheet Creator