Permissions, Permissions, Permissions!
Maybe one of the most inportant pluggins ever.
Your bukkit server is going to need this if you want to separate your users on groups.
And when I say on groups I don't mean OP's and no OP''s I mean groups like: builders, users, peasonts,VIPs, Moderators, Half-OPers, vampires, orcs, elfs, humans, undeads... basicly - whatever you want!
Permissions lets you create a custom group, that you can add people to, with certain permissions that you give them.
Download link: http://forums.bukkit.org/threads/gen-sec-econ-tp-admn-essentials-v2-6-2-a-collection-of-useful-commands-and-features-1185.15312/
Enjoy my friends!