Bukkit

Allow players to redeem items or enter codes for in game items

NeedTown - Notify your server of players looking for a town or faction.

Give's hundreds of different particle combinations for players on your server

The InternalPluginManager is a facade for Minecraft server mods to provide cross-platform plugin comptaibility.

JoinFirework

lets you change your server MOTD with the players username, as well as give a random motd

BlockThatName blocks all spaced, and strange character names!

Simple Bukkit Info

Changes Environmental Damage Values e.g. Cactus and Fall Damage

A plugin that lets admins chat privately together.

dynmap addon for adding landmarks

Prevent players from placing fire and lava while invisible!

MapReplay

Create bounty signs and reward players when the target is found.

InvisibilityDust

Commands-Plus

A simple plugin that blocks the user from creating a nether portal unless they have permission.

Allows admins to send gifts to players though and easy-to-use interface.

MultiSpawns allows you to have multiple spawn points set for your server.

Stop users from PVPing while in fly/god mode in Essentials