Paint.NET

[CoDeX]

Feature:
Simple, intuitive user interface

Every feature and user interface element was designed to be immediately intuitive and/or quickly learnable without assistance. It is also designed to be immediately familiar to users of Paint and Photoshop.
Layers

This allows you to compose one complete image from a stack of other images that are blended together.
Unlimited History

Every action you take on an image is recorded in the History window, and is only limited by available disk space and memory.
Support for many popular file formats

The following formats are supported: PNG, JPEG, BMP, GIF, TGA, DDS, and TIFF. The native Paint.NET image type is PDN which preserves layering.
Effects and Adjustments

You can apply many specialeffects to your image, and support for image adjustments is also possible.
Full Support for Dual- and Quad-Core CPU's, and 64-bit

Paint.NET is fully optimized for the latest in processor technology enabling dramatic performance improvements and the ability to work with larger and more images.
Tablet PC Support

The Paintbrush tool supports the Tablet PC's pressure-sensitivity feature, allowing you to draw in a more natural and artistic manner.
Extensibility

Paint.NET supports plugins that add new effects, adjustments, and file format support.
Free

Paint.NET is provided for no cost with generous licensing terms.