I am Andreas Loew, developer of TexturePacker. I recently added support for CEGUI's ImageSets.
TexturePacker grabs a directory of images and creates one big set from them. It also supports several optimizations, dithering and export to formats like RGBA8888, RGB565, RGB5551 and pvr format for iOS devices.
Most functions are free to use - some advanced however require a license.
TexturePacker is available for Windows, MacOS and soon for Linux.
I am open for all kind of feedback
Cheers
Andreas