GraphicsMagick Image Processing System
GraphicsMagick has been one of the hidden gems I've found over the years in my career.  HN would probably mostly be interested in it for server-side applications [+], but it is an absurdly useful thing to have sitting around your laptop, too, for those times when e.g. you've got a screenshot of something but a government/etc site will only take a file upload in pdf format.  "gm convert foo.png foo.pdf" and you're done.[+] Really good for common "take an arbitrary user upload; grab info to crop it from your web app; crop and resize to appropriate dimensions" sorts of tasks.