Probably one of the most used open source image applications, GIMP has a well-rounded list of features, and there is even a tutorial on how to setup GIMP to mimic Photoshop’s configuration. Some of the features GIMP has includes:

• Painting tools including brush, pencil, airbrush, clone, etc.
• Gradient editor and blend tool
• Unlimited number of images open at one time
• Full alpha channel support
• Layers and channels
• Editable text layers
• SVG path import/export
• Transformation tools including rotate, scale, shear and flip
• Supports all of the major file formats

2. Paint.NET

Paint.NET started as a Microsoft-mentored project by a college senior and is no longer open source. But it’s free and has a lot of features that make it a good alternative if you are working on a Windows machine. Features include:

• Simple, intuitive, and innovative user interface
• Layers
• Effects such as blurring, sharpening, red-eye removal, distortion, noise, and embossing
• Adjustable brightness, contrast, hue, saturation, curves, and levels
• Simple tools for drawing shapes
• Unlimited history

3. Inkscape

Inkscape is an Open Source vector graphics editor, with capabilities similar to Illustrator, CorelDraw, or Xara X, using the W3C standard Scalable Vector Graphics (SVG) file format. It supports many advanced SVG features (markers, clones, alpha blending, etc.) and great care is taken in designing a streamlined interface. It is very easy to edit nodes, perform complex path operations, trace bitmaps and much more. Like Illustrator, Inkscape offers a wide range of object-based tools and functionality:

• Drawing tools including pencil, pen and calligraphy
• Rectangle, star, spiral shape tools
• Text tool
• Moving, scaling, rotating, skewing transformations
• Layers
• Gradients and patterns
• PNG and PostScript export
• Perfectly compliant SVG format file generation and editing