iResizer is a very intriguing app for photographers using Macs. The app is US$19.99 on the Mac App Store.
Have you ever wanted to change the aspect ratio of a photo? Maybe take a 16x9 photo and make it square for Instagram? If you squeeze the photo down, you will wind up with your subjects looking pretty distorted. iResizer attacks that problem in a way that even Photoshop can't do without a lot of steps.
To make it work, you import your photo. iResizer opens TIFF and JPEGS, but not RAW files. Use the brush to mark objects that are your subject. It could be a tree, people, anything you don't want to be distorted. Then choose an aspect ratio and you get a new photo in the proportions you want.
The software also does something Photoshop does well, and that is it can remove objects or people from a scene. Mark what you want to lose, then resize. Your selected object will be gone, and your picture will be at a new aspect ratio.
I tried the app on several photos. In the photo example above you can see how I removed one of the buttes in scenic Monument Valley, then resized to proportions I found more appealing. It all worked quite well.