Here was a quick one from a request on the Paint.NET discussion board.
The user wanted to blur the edges of an object by using the primary color instead of the object color (similar to the Feather Filter that I had previously written).
You can download the precompiled effect DLL here: Halo.dll
Just drop this file in your \program files\Paint.NET\effects directory and you should be all set.
As I really only changed a few lines of the Gaussian Blur function, I'm going to skip posting all of that code--its pages and pages long! If you really NEED to know how I did it, just email me and I'll get back to you.