From Flying Meat:
Built on a foundation of simplicity, Acorn eschews the outmoded "many palettes" design from yesterday's editors. Acorn presents a fluid new interface that will be familiar to users of Apple's iLife applications. Designed for simple editing, Acorn features flood fill, gradients, vector shapes, drawing and erasing tools, and more. It also brings a few new tricks to the table, including chaining filters and easy image and canvas resizing. From quickly adding text to an image to creating a new graphic with pixel-precise control to just sketching out ideas, Acorn is great for all kinds of image editing.
What's new in this version:
New Automator Actions: Scale and Export to Folder. This makes it super easy to batch process a ton of images, and have them exported to your favorite format.
Various OS X 10.11 El Capitan workarounds and fixes.