Bukkit

Vault is a Economy/Permission plugin for hooking into the various Economy and Permission plugins.

WorldGuard

A collection of useful commands and features for server admins and players.

EssentialsX

Plugin Manager For Bukkit

Allows server admins to easily modify how Minecraft works without programming anything

Tablist

A plugin which teleports you to your set home.

Fully customize any mobs (and create custom mobs) with a powerful scripting skill system, attributes, equipment, special AI, and much more, along with custom spawners and spawning rules. The ultimate plugin for making RPG servers and minigames.

Change your ingame skin

An mcMMO themed plugin for horse role-playing.

SetSpawn

mcMMO

Gives the ability to cut down trees at once. Just like Timber mod.

A plugin to warn your players when your server will restart!

Mobs will be teleported up until they are no longer suffocating

Change the colour of the server chat!

Light weight and easy to use!

If you've ever used the VeinMiner mod for minecraft Forge, it has now been recreated for Bukkit, CraftBukkit, and Spigot

BukkitGUI - the most downloaded GUI for bukkit and any other server