Spyer is a pack of two plugins. One is SpyerAdmin, which can simulate players leaving, making them invisible and removing them from known player lists and commands. The second is SpyerFun, which allows normal players to use configurable items to become invisible. The plugin removes an item from the stack per n milliseconds.
Here is something the users have done using this plugin:
Go to Installation & Configuration for installation and configuration instructions!
Sourcecode can be found on github.