Bukkit

Simple, easy-to-use Reserve slot plugin

Highlights vanilla structures and features such as Fortresses, Spawn Chunks etc

Allows server admins to create their own inventory menus to perform scripted commands in a similar manner to command blocks

A simple and easy to use bank plugin, providing Bankaccounts, Interest and Loan.

Create colored prefixes and suffixes easily without ANY hassle!

Gives the player a Diamond upon Logon!

allows you to see commands being used by other users.

Double the depth of your world

VoteCheck - Vote Reward Plugin (Votifier and MinecraftHangout.com)

Show information about your server live on your desktop.

Create your own Coupons

Randomly-generated treasure hunts based on values you set and as often as you want.

This plugin adds classes/teleportation to the game.

A coin-based economy plugin with multiple denominations

SimpleTrails

Allows setting of players health, air, and hunger

Write JavaScript scripts that can interrogate and modify the world and bind them to in-world events.

High Speed Rails to make minecarts worth building

Blocks advertisemen and spam

Bans player on death.