Piwik is a free and open source web analytics application written by a team of international developers. It runs on a web server with PHP / MySQL. Piwik is used by more than 1.000.000 pages, equivalent to 1,2% of all existing websites, and has been translated into more than 45 languages. New versions of Piwik are regularly released every few weeks.
Piwik displays reports regarding the geographical location of the visits, the source of the visits (that is, if they came from a website, directly, etc.), the technical capabilities of the equipment used by the visitors (browser, size of the screen, operating system, etc.), what the visitors did (the pages they saw, the actions they took, how they left the website), the time of the visits and much more.
In addition to these reports, Piwik provides some other features that can help users analyze the data that accumulates in Piwik. What's more, developers are encouraging users to extend the platform using Piwik APIs, and many are already doing some creative stuff.
Open Web Analytics
Open Web Analytics (OWA) is a completely open source web analytics software created by Peter Adams. OWA is written in PHP and uses MySQL as a database engine, being possible to run it under a range of AMP solutions (Apache + MySql + PHP) on different web servers (Apache, Nginx, etc.). OWA is comparable to Google Analytics, although the biggest difference is that anyone can install and run it on their own server, while Google Analytics is a service offered by a monopoly (Google) for purposes that some believe may be unsanctified. OWA brings support for WordPress and MediaWiki, two very popular content management systems that are also open source.