Photo Magician is a nice freeware for batch image resizing. The little tool comes with a bunch of presets for various devices, just like iPhone, PSP, Zune), Nintendo DS and common screen resolutions (800x600, etc.). Photo Magician also allows to scale to a custom size or percentage, exclude images under a certain size and swapping formats. As addition, the freeware process the sub-folders inside your input folder. All in all, the convertings are fast, and the quality of the resized images is very nice.