A newer version of Paint.NET is available.Or, Learn More About Paint.NET
Full user review
"Almost as good as Photoshop, I have to say"
If you can learn to use Paint.NET efficiently, it is much better than any free image editing software out there, even Gimp imo. It also allows for user created plugins. I used this program to create some pretty convincing edits to images, and they actually look real to the point it's scary.
The user interface, especially the layers, can seem a little overwhelming at first. I was expecting this to be almost the same as MS Paint which came with Windows as the name implies. But this is a full blown program. But if you learn to use it, it's really effective.