Bukkit

Player applications for ranks, based on criteria. Applications can be rated by users with permissions and are approved by staff members.

A multi-channel chat-system. Can be used as an open-source, and more light-weight alternative to plugins like Herochat.

Day/Night - PvP & Difficulty Cycle

ItemCase

Namer Ability to nickname yourself and others (FOR EVERYONE)

BasicQuests implements randomly generated quests with rewards for players.

Actor allows you to create NPC actors which playback pre-recorded actions in world.

The best backpack plugin available

A Plugin That The Player What The Server's YouTube Channel

SetSunny

Save inventory and experience on death, implement death taxes, etc.

GoGoBanYourself

Lightweight, fully customizable, server essential.

Open Iron Doors by Rightclick

Easy using interface, like chestcommand.

A simple Minecraft surgery plugin where players use a scalpel to open a chest GUI with organs.

Sewers

[LuckyBlocks] MC-1.10 + API

Player+Mob vs Player+Mob

Simply Blocks All CAPS Messages