Open Web Analytics - Analytics used by Wikimedia

Open Web Analytics is open source web analytics software that you can use to track and analyze how people use your web sites and applications. OWA provides web site owners and developers with easy ways to add web analytics to their sites using simple Javascript, PHP, or REST based APIs. OWA also comes with built-in support for tracking web sites made with popular content management frameworks such as WordPress and MediaWiki.

Track custom site actions and events that you define. Logs new events to a log file or queue instead of writing directly to the database. Caching support is provided using Memcache.