Hibernate is a very simple plugin I use in all my servers. It will reduce CPU usage to about 0-1% when no players are online on the server. This can be used for a lot of purposes.

This plugin will only be useful for small servers that tend to be empty at night or during schooldays. If you've got 500 players, the plugin will never "enable" and will not make any difference.

Features

Save electricity (if you host the server)
Let other software use the CPU when no one is playing Minecraft.
Allow more servers to run on your machine
Works for all versions of Craftbukkit or Spigot
Very lightweight plugin. Does not cause any lag at all.
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How does this work?

The plugin will intentionally "lag" your server while no players are connected. You can see it as freezing the server. It'll make it dormant until a player connects.

Example

Server with no players online, without Hibernate:

Server with no players online and Hibernate installed:

Updates

This plugin requires very little updates. It should work on all Minecraft versions newer than 1.7.x. If you have remarks on this plugin, please contact me on SpigotMC as I'm not very active on Bukkit any more. https://www.spigotmc.org/resources/hibernate.4441/