Bukkit

RCON allows access to console from in-game.

MMOCraftPlus Bukkit Plugin

PM'ing Simplified and Improved

TheMolkaChat

Chest GUI with multiple links.

Some extra commands/functions for a server

Change your chat prefix | NEW PLUGIN!

LogStream

MSGName Reloaded

A Easy To Use Autobroadcast

ColoredChat

This plugin allows players to "record" the chat. — Part of the Samaritan Suite

LogBlockStats - lastseen and onlinetime functions

StaffOnline

Let players use hashtags like on Twitter in the chat!

NoPermsMessage

Server Broadcaster allows server owners to broadcast important messages or reminders across the server for all players to see.

MaxPermission

Inform your players about the current voting sites!

CraftXMPP