G'MIC is an amazingly powerful image processing tool for the command line (or via GIMP, if you install the plugin). It's aimed very much at experts, but has features which can be used by just about everyone.
At its simplest, you might use the program to view a series of images (gmic pic1.jpg pic2.jpg pic3.jpg) or convert an image from one format to another (gmic ThisPicture.png Output.png).
G'MIC also supports an array of image filters and processing commands. You're able to mirror, crop, rotate and resize images, blur and sharpen them, remove red-eye, use various warps and deformations (fisheye, sphere, kaleidoscope, more), apply top-quality paint and drawing effects (as good as those in any image editor), create animations, and add captions, borders and drop shadows. (And that's just a tiny fraction of what's on offer.)
These effects are sometimes very easy to use. Applying a sepia-type "old photo" effect just requires the command gmic image.jpg --old_photo . But G'MIC also provides astonishing flexibility, for people who need it. You don't just have a basic "--blur", for example: you're able to select a filter type, a border setting, various blur types (angular, radial, linear, selective), a direction (x/ xy/ xyz/ y/ z) and more. And these can be combined on a single line in whatever order you like.
The results can be amazing. G'MIC isn't just another "me too" image tweaker, it's easily strong enough for professional use, and you don't have to take our word for it: just browse through Flickr's G'MIC group pool and see for yourself.
But even this is just the start. G'MIC can also work with videos, plot 2 or 3D functions, create 3D logos and models, display a filtered webcam stream, and more. It's a lengthy list, but fortunately the program has some built-in demos to show you what's possible: enter gmic --demo and prepare to be impressed.