GroupManager was one of the first permission plugins ever available for Bukkit. Although there are plugins such as LuckPerms with more functionality than GroupManager, and the official support and development of Essentials plugins has ended, GroupManager is still preferred over other plugins by lots of server administrators.
Please visit our Spigot page for more info: https://www.spigotmc.org/resources/groupmanager.38875/
Maven repository: https://raw.github.com/GroupManager/GroupManager/maven