Paint.NET is free image editing and photo manipulation software designed to be used on computers that run Windows. It supports layers, unlimited undo, special effects, and a wide variety of useful and powerful tools. It started development as an undergraduate college senior design project mentored by Microsoft, and is currently being maintained by some of the alumni that originally worked on it. Originally intended as a free replacement for the MS Paint software that comes with Windows, it has grown into a powerful yet simple tool for photo and image editing. The programming language used to create Paint.NET is C#, with a small amount of C++ for installation and shell-integration related functionality.
Paint.NET can be used to enhance and clean up your photographs. Using the Clone Stamp tool, the power lines that criss-crossed in front of the Space Needle were carefully removed. Other operations were performed that had the effect of smoothing out the image without blurring it, adding some "bloom" (glow), and increasing the contrast.
FEATURES:
Simple, intuitive user interface
Every feature and user interface element was designed to be immediately intuitive and quickly learnable without assistance. It is also designed to be immediately familiar to users of the original MS Paint software that comes with Windows.
Layers
Usually only found on expensive or complicated professional software, layers form the basis for a rich image composition experience. You may think of them as a stack of transparency slides that, when viewed together at the same time, form one image.
Powerful Tools
Paint.NET includes simple tools for drawing shapes, including an easy-to-use curve tool for drawing splines or Bezier curves. The Gradient tool, new for 3.0, has been cited as a huge improvement over similar tools provided by other software. The facilities for creating and working with selections is powerful, yet still simple enough to be picked up quickly. Other powerful tools include the Magic Wand for selecting regions of similar color, and the Clone Stamp for copying or erasing portions of an image. There is also a simple text editor, a tool for zooming, and a Recolor tool.
Unlimited History
Everybody makes mistakes, and everybody changes their mind. To accommodate this, every action you perform on an image is recorded in the History window and may be undone. Once you've undone an action, you can also redo it. The length of the history is only limited by available disk space.
Read More/Download/Screenshots
Here.
It requires .Net Framework 2.0 and there is a link on the site.
Paint.Net Version 3.05 
