These plugins are mainly Alpha Level Utilites.
EditBy: These plugins are mainly Alpha Level Utilites for removing particular levels of color for color correction.
Alias
Submenu: Effects -> Object
Alias is a multipurpose tool originally created to remove any transparent Pixels from an object before applying BoltBaits very powerful Feather plugin as Feather was causing distorted results when applied to already existing antialiasing

Color Trimming
Submenu: Effects -> Color
Color Trimming is used for color correction in photographs when too much of a particular color is present on the applied image. It will remove a defined range of that particular color allowing for more vibrant colors to be displayed with in the image.

Input to Output
Submenu: Adjustments
Input to Output allows you to adjust the value of one color channel (The input) by the value of another channel (the output). Very useful for making alpha masks, removing color, etc.

Brightness/ Darkness to Alpha
Submenu: Effects -> Color
This is an old plugin of mine whose functionality is useless compared to a Tanel plugin however its simple user interface can still make this plugin useful. It allows you to remove dark or bright parts of an image and turn
them into Alpha.

Invert Alpha
Submenu: Adjustments
Similar to Invert Colors, this plugin inverts the Alpha channel.

About Silhouette Plus
Submenu: Effects -> Color
Silhouette converts the entire canvas into your selected color while preserving the alpha levels of each pixel. As well, you can also specify an alpha range in which to silhouette. This is usefull for object manipulation and for testing the transparency of the canvas is particular areas. you can free download Mike Ryan's Plugins 1.0 now.