These plugins are very simple. Each one is stored in a single text file. They have associated icons. To create a .dll for one of these plugins, load it into CodeLab (another plugin for Paint.NET) and compile them, making sure to include the icon.
Fuzzy Edge Detect: applies a stylistic edge detection algorithm to the current selection.
Channel Ops: provides unary operations on channels, channel swapping, and inversion.
Delete Selection: deletes whatever is selected, but doesn't finalize (remove) the selection at all.
Replace Colors: replaces the chosen color with another, with tolerance to select nearby colors in RGB or HSV ranges that can be limited.