HeadBlocks is a plugin for the famous Minecraft.
More precisely, it's a server plugin compatible with Bukkit.
This plugin allows you to add a block on a player's head.
It seems useless but on a multiplayer server it can be a good way to recognize teams for example.
To use it, you just have to copy the following .jar file : HeadBlocks.jar in the "plugins" folder of your Bukkit server.
For more information see the project on GitHub.