Preview’s sidebar is a great tool for sorting images before you commit to importing them into your image editing app (say, iPhoto). For example, if your friend hands you a memory stick of 100 images, make three folders on your desktop named “originals,” “keep,” and “toss.” Drag the files from the memory stick into the originals folder and open them in Preview. Next, drag thumbnails from Preview’s sidebar into the appropriate folder (“keep” or “toss”). The original files will be left intact, but copies will appear in your sorting folders. When you’re done, delete the originals folder.