This is my collection of Paint.NET effects thus far.
A quick rundown of the effects in the DLL:
Mirror over line
This lets you draw a line on a thumbnail of the image in a dialog and reflect what's on one side of the line over the other.
Conditional Hue/Saturation Adjustment
The conditional hue/saturation can adjust the hue or saturation of a portion of the image based on the existing hue values of pixels.
Below are some before and after shots.