Windows: If you're not about to manually convert that pile of images in front of you but you've found the batch converters you've tried to be lacking, free and portable Photo Magician offers both fine tweaking and drag-and-drop simplicity.
Photo Magician has two modes: full and quick convert. In the full mode you select an input and output folder and options like whether or not you want to scan the sub folders, overwrite the originals, speed up conversion by ditching the image preview, and unify the image format to a format of your select, among other options.
Photo Magician also supports presets covering popular portable devices and common image sizes. You can select Custom to set your own sizes if they aren't covered by the presets but unfortunately you can't save the custom presets you create, an oversight we'd love to see corrected in future versions of an otherwise strong program.
Full conversion mode aside you can also click "Quick Convert Mode" in the menu bar of Photo Magician and the program will minimise to the magician's hat — see at left here — like a sidebar gadget. Drop Photos right onto the hat and they will be automatically converted and saved after being reduced by the percentage you've selected.
Photo Magician is portable freeware, Windows only. Have a favourite tool — image-related or otherwise — for batching tasks? Let's hear about it in the comments.