Essentials

Essentials is a well established plugin which brings a variety of useful features to the game. Such as

There is an in depth guide here..

http://dev.bukkit.org/server-mods/essentials/

But if all you are looking for is a command reference then here is where it is

http://wiki.ess3.net/wiki/Command_Reference

NOTE: Only a selection of the essentials commands are available to normal players on the server as many are reserved for Moderators or above rank.