iOS: Pixelmator is already a pretty great photo editing app on iOS, but the most recent update takes advantage of the new 3D Touch feature on the iPhone 6s in just the way you'd expect it to, with touch-sensitive painting. Alongside the hard-pressing on the icon for shortcuts and various peek and pop features, the newest update to Pixelmator allows you to hard press while painting to make thicker lines, or touch softly to make lighter lines. While this doesn't exactly make painting on your iPhone a delightful experience, it does help make it a little better. Beyond that, the update also adds in support for the iPad Pro and the Apple Pencil. For everyone not on Apple's newest devices, you get a bunch of new artist-designed brushes (included a pixel brush for anyone who loves pixel art), a new auto-straighten feature, and improved cropping.
Pixelmator ($7.99) [iTunes App Store]