Paint.net is a free and very capable image editing software for Windows. Great alternative to Photoshop for people that do not need all the stuff PS offers.
ImageJ is a public domain, Java-based image processing program developed at the National Institutes of Health. ImageJ was designed with an open architecture that provides extensibility via Java plugins and recordable macros.
Photoline is a professional image editor which offers creatives a mostly non-destructive workflow for 8/16/32bpc image processing and compositing in RGB, CMYK, and Lab modes.
Full vector editing, DTP features, and a HSV/HIS workflow are supported.
openCanvas, popularly abbreviated as oC, is a popular Japanese image editing program.
Image Enhancer is a desktop application based on Image Processing concepts. A set of total 64 basic and complex Image Processing filters are implemented here. Moreover snapshots can be captured using video surveillance camera/webcam attached with the computer.
Everyday drawing There are hundreds of painting and drawing apps on the App Store. This one is a bit different. It doesn’t want to grow up to be Photoshop, Illustrator, or Draw Anything the Next Generation. It’s not intended for professional artists.This app doesn't have any features at the moment.