Those are not plugins they are mods, mods are personal things that you can only see in singplayer and have to be altered into your minecraft folder. Plugins however are on a huge folder called plugins in the server folder and they can be used in multiplayer. So many times I see mod suggestions for the server when we can't use them as they are for singleplayer.
When Yogscast make a video together in multiplayer about a plugin I can use it!
When one of them makes a video alone in single player that is a mod, this means Simon or Lewis have to play it alone. Sometimes Lewis plays the mod and Simon watches and does the voice over with him.