Paint.NET is a fairly powerful, but free image editing program. It is one of the programs that I often use for making games, simply because it's free. One of the features of Paint.NET that I have really come to love is its plugin system. It is extremely simple and powerful.
If you want to learn how to use plugins in Paint.NET, or even how to make your own plugins, check out this link, which is extremely well put together: http://www.boltbait.com/pdn/codelab/
I've made a couple of plugins for Paint.NET, mostly so I could perform the stuff in this Photoshop tutorial: http://www.voidix.com/starburst.html
Anyway, I'm uploading my effect plugins here, mostly so that I can download them when I'm using a different computer, but if you think you can put them to use, feel free to download them.
Similar to the wave filter in Photoshop. Seems to work fairly well, but I haven't tested it with a selected region, only the entire image.
Converts your image from cartesian to polar coordinates. I haven't really tested it with a particular region selected, so who knows what'll happen. I tried it with a non-square rectangle, and that didn't seem to work so well either. It can't convert back to cartesian yet… sorry.