Core is a plugin created by EndyFreaky a.k.a Endernation.You can download the plugin at http://dev.bukkit.org/bukkit-plugins/core Remember though that this is not the latest version!The latest version of core can be downloaded at http://dev.bukkit.org/bukkit-plugins/core. You want older versions? Goto http://dev.bukkit.org/bukkit-plugins/core/files