Bukkit

This plugin uses 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
You can disable the stat collection via /plugins/PluginMetrics/config.yml if you wish.