Bukkit

You race pigs on saddles.. What else is there to say?

Champions

strike people with lightning

ImproveMob

ChanceCraft

YouNeeds

Touhou Player Enhancements

Tame those Endermen!

User registration and nickname protection with OpenID

A simple mob spawner plugin

Channels based, configurable, permissions respecting chat plugin.

Valve

City simulator

Provides basic functions to inform your players if the server is under maintenance.

RolePlayMobs - WARNING Its just an Idea

PailStone extends the capacity of users to interact with redstone and vice versa.

A Quick Way Of Notifying Users When They Can Vote Again

Counter Craft

Prevents dragons from spawning

BlockNotifier