Paint.NET is free image and photo editing software for computers that run Windows. It features an intuitive and innovative user interface with support for layers, unlimited undo, special effects, and a wide variety of useful and powerful tools. An active and growing online community provides friendly help, tutorials, and plugins.
I know this app has a few fans on here, but I've never tried it.
They keep improving it. I still use it more than IcoFx for making icons using the IcoCur plugin (I use an older version because the new IcoCur just crashes on my system).
I've tried it, but I don't like the blur brush function thingy missing in it since I need that feature in work etc. To say the least, it's an incredibly easy-to-understand feature-rich raster graphics editor for the masses.