Halide iPhone (and iPad) camera app, for more control while shooting iPhone photos.
Darkroom for quick editing iPhone photos within the camera roll.
Adobe Lightroom for IPad, for a richer set of editing tools, and for importing RAW files from my Sony camera.
Low-Learning-Curve Mobile Apps
I use PicCollage (subscription) for making collages. It comes with grids and other templates, and lets you add text and stickers. It’s limited in what it can do, for better and for worse. For me, for now, I appreciate the limits.
Sometimes I want to take a few screenshots from my phone and paste them together, to share an article or UI that’s taller than will fit in one screenshot. PicSew has a very simple UI. It’s also got tools to scrub your menu bar replace the time and cellular or WiFi network with generic names.) I only bother with that if I take a screenshot when my battery is low, since I hear that that kind of screenshot distracts some people and even makes some people anxious. (How’s that for mirror neurons?)
Retouch removes power lines, bits of trash on the street, and other small objects from a photo. You can use it within the Photos app; it modifies the photo in place, while leaving the ability to restore the original.
These are all mobile only, I think. I don’t have good desktop alternatives for these. I typically just whip out my iPad (or use the phone for very quick low-quality work).