The list would never be complete unless we mention “GNU Image Manipulation Program” or GIMP for short. In my opinion, it is the best Photoshop alternative you’ll ever find.
The software supports a wide range of operating systems and file formats. It also gives you access to a full set of tools not unlike those features found on Photoshop.
With hundreds of effects and overlays, as well as the usual image editing functionalities, Pixlr is indeed a great free alternative to Photoshop. Of course, you’ll need a good amount of RAM because this will definitely double your Chrome/Firefox memory. Pixlr is also available for iOS and Android.
Paint.NET was originally intended to be an advanced version of Windows Paint. It’s now an open-source project maintained by some of the alumni of the original undergrad project sponsored by Microsoft.
Seashore is based around GIMP’s technology, although not quite as powerful. It aims to serve the most basic image editing needs, so if you’re not doing anything advanced, Seashore is perfect for you.