Bukkit

Survival with the ability to... levitate blocks? [Telekinesis]

ManHunt plugin from Dream's videos (even better)

A plugin to access iron doors and iron trapdoors with custom keys.

Makes an arrow to emit light around it and spawn a torch on impact.

uPlanes - Planes in Minecraft!

Mayhem Tower Defense

CodeBlocks 2

RadioTowers

Adds role-playing functionality to your game using factions!

A plugin which allows double exp to be given to certain people, with custom multiplier

Multiworld management made easy

Get drunk with your friends!

PumpkinStep

Adds a few useful mechanics to minecraft

Makes cobwebs into barbed wire by damaging player.

A plug-in that adds multiple jumps pads to the game.

Add custom recipes to Bukkit via a simple config interface.

RailSwitcher

A customizable grapple hook!

Prevent players in freecam from interacting through walls.