The following screenshots show the plug-in and the modifications it makes to the Winamp skins preferences page along with the additional right-click options for skin entries allowing you to generate a preview (classic skins only) or to rename the skin to a more consistant naming pattern. This shows the submenu filtering for the different skins (what was the Skins In Submenu functionality) though there is additional control on how they are handled (see below).
The related forum thread for this plug-in is here. With the plug-in installed you can place classic and modern skins into their own submenus in the main skins menu (from the Skins In Submenu plug-in) to being able to filter the installed skins when searching for skins in the preferences. The aim of this plug-in is to make it easier to manage your installed skins.
