imagemagik is pretty good - but uses the terminal not gui, multiple formats, allows you to change formats, add, remove, convert animated gifs to sprite sheet etc. you need a bit of unix knowledge to put together your command line scripts, but there are examples on the site as well.
I am using the slice tool in photoshop to slice up images for a puzzle game in a 6x8 grid. Once it is all set up I can use the same slices again and again.
Comments
http://www.imagemagick.org/script/index.php