From HPC Technologies:
GAPS is an image sorting and viewing application written to deal with enormous folders full of images. It can easily handle hundreds of thousands of images per folder and continue to operate quickly and stably. GAPS' DuperFinder feature can scan all of your image folders for duplicates, shows you thumbnails of the dupes as confirmation, and easily removes the extra copies to save disk space. With GAPS' image sorting features, you use your keyboard to quickly sort images from an "Incoming" folder into multiple folders of your choosing. You can sort images one at a time or in batches by using your mouse to select multiple images (shift-click). As you sort each image, the file is moved and the next image in your incoming folder is displayed automatically. If you make a mistake, GAPS' Undo feature allows you to go back and change your mind about deleting or categorizing an image. GAPS works best when you use the keys on the numeric keypad to sort, but you can assign any and all keys on your keyboard to perform any sorting action you like. When you don't feel like sorting images, GAPS' slide show (including full screen) displays all the images in your collection either randomly or in order.