Bukkit

CFAuth is an authentication system for offline servers. Secured with MD5, it's the most lightweight one out.

A plugin with stuff (commands) that might be useful I think

Display messages in Minecraft

A very massive chat plugin

RespawnHelper helps you automate minigames by allowing to run multiple commands instantly at a configurable respawn point

Allows users to maintain a friend list.

Plugin for logging user join/connects

Let the blocks drop that you mine in creative

API for all LinPlugins and some plugins made by jacklin213

A simple utility plugin for spigot

OnAndOff

This plugin makes the administration of your MineCraft server easier.

Disables /plugins for players who lack a permission node.

Makes plugins use their logger

An API for saving custom data (For Blocks, ItemStacks, etc.)

Fires an event when an arrow hits a block, and provides the block that was hit

Memory Usage Viewer for Bukkit, Spigot and PaperMC

API to send email and text messages to players!

A plugin for runtime scripting using Jexl

Allows you to change the dispenser behavior