Bukkit

The Metrics system is a way for plugin developers to track statistics collected about their plugin, shown on mcstats.org.

The pieces of information the mcstats sends are: the plugin name and version, what kind of bukkit it is being run on (original, modded, those things like Spigot or MCPC builds), java version, os versions (windows 7,8,etc Mac, Linux, etc), number of players, country of the server. These help the developers (me) know on what kind of systems the plugins are being used. The data is only displayed in an aggregated form and no personally identifiable information is collected.
The pieces of information that mcstats does NOT collect: Server information like IPs, names, and other related data is not collected.

If you would like to opt-out of the metrics data collection, you are free to do so. The steps are outlined in the quote below and on the learn more link below.

To view the metrics information about this plugin see: http://mcstats.org/plugin/HopperFilterSimplified

• NOTE: What you see on the site, is exactly what I can see. I use the above url as well to view stats.
  

To view the exact code I have in my plugin that hooks into mcstats.org, see:

• Look on about lines 76-82 of: HopperFilterSimplified.java
  
• The entire file from MCMetrics.org that I use: Metrics.java
  

For a list of the top plugins that use mcstats, see: http://mcstats.org/plugin-list/

For more infomation, see http://mcstats.org/learn-more/

Quote from from mcstats.org learn more link above:

Server Owners

For this service to track a plugin, an author must explicitly add it to their plugin. They will most likely have made note of this addition in any changelogs. Your server cannot be identified nor controlled in any way by MCStats. As well, you have access to the same data that the plugin author can see.

You are free to opt-out of submitting data whenever you wish. This will immediately stop sending data for any plugins that supports MCStats / Plugin Metrics. Simply edit plugins/PluginMetrics/config.yml and change opt-out: false to true

If you have any questions at all about how this service operates or anything else, please do visit us in IRC or email me directly: hidendra [at] mcstats.org