Open Web Analytics (OWA) is an open source web analytics framework written in PHP, that provides a generic set of PHP and HTTP APIs that application developers can use to integrate web analytics into any application. The Framework also has built-in support for popular web applications such as WordPress and MediaWiki.

More info: WiKi

Karim Vaes released OWA Most Popular - a very cool Wordpress widget that uses OWA to put a list of your most popular posts/pages in the sidebar of your blog.