Bukkit

SquirrelVoteParty

MCGuns: a gun plugin in minecraft!

Economy Plugin which uses SQL and can be used for webshops etc

A fun shoot'em down plugin

MineChest is a server chest plugin for auto-sorting.

Ever wanted to add Ghosts in your game? This plugin allows you to become a ghost yourself!

Allows you to join and create factions

Control what players can do while flying

Global Economy for Minecraft

Allows administrators to disable certain features of Minecraft.

PowerToolsPlus

ReserveMe - Let your own players reserve their own slots!

Permits on login remapping of group membership.

Make your Nether to become easier!

Let players teleport themselves back to where they last died!

The Ultimate Statistics tracking plugin

Tradable and simple minecraft prepaid card

A powerful private chat system for interactive non-intrusive messaging

XSkript is a Bukkit plugin which allows server admins to customize their server easily, but without the hassle of programming a plugin or asking/paying

Use a Twitter Feed as your announcements!