Paint.NET
Spectacular
Full user review
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"Very good for its purpose"
Pros
Very nice GUI, almost similar to Photoshop. Easy to use and works like a charm. Great for those last minute touch-ups that require more than some auto-adjust. Its simple interface makes it a great pick for anyone who wants a free image editor, without to over-kill and clutter of GIMP.
Cons
Limited on features, as opposed to GIMP. Windows only, and not portable :( .
Summary
An excellent choice for anyone who wants an easy to use image editing interface, without the clutter and confusion of a complicated professional image editor. Does what you need easily and quickly, without too many features, though this may be a good or bad thing depending on the project.