Piwik is an open source (GPL license) Web Analytics software. It generates reports on website visitors, popular pages, the search engines keywords used, and the language spoken.
Piwik’s features are built inside plugins: one can add new features and remove the ones not needed. Developers can build their own plugins. Users also own the data. Once Piwik is installed on a server, the data is stored in users’ own database and statistics can be obtained by using open APIs (publishing the data in many formats: xml, json, php, csv).
Additonally, the user interface is fully customizable, users can drag and drop the widgets they want to display and create the reports they need.