Bukkit

This plugin utilises Hidendra's plugin metrics system, which means that the following information is collected and sent to mcstats.org:

• A unique identifier
  
• The server's version of Java
  
• Whether the server is in offline or online mode
  
• The plugin's version
  
• The server's version
  
• The OS version/name and architecture
  
• The core count for the CPU
  
• The number of players online
  
• The Metrics version
  Opting out of this service can be done by editing plugins/Plugin Metrics/config.yml and changing opt-out to true.

It also uses an auto-updater! To disable it change in the configuration

EnableUpdater: false

The plugin automatically loads down the latest version. This is not dangerous because the plugin is previously checked thoroughly by the Bukkit team.