First time released in 2004 and designed to be an alternative for Microsoft Paint (Paint.NET and Microsoft Paint are two different products, even names are similar), this tool has evolved continuously and today allows a variety of image editing features.
Is working with layers, supports tabs, features special effects such as blur, sharpen, red-eye, sepia, adjust contrast, hue and many more. Is not just a simple program, supports complex operations and is simple to use. Other popular tools included are clone stamp, magic wand, text editor and gradient generator.
Have an active community and many plugins available, including one to work with PSD files.