Piwik has launched as open source web analytics software under a GPL license that gives its users detailed reports on their websites pageviews, visitors, search engine keywords, popular pages and more. Their goal is to be open source web analytics software that is a good alternative to Google Analytics.
Some features from their site:
•    The features are built inside plugins: you can add new features and remove the ones you don’t need. If you are a developer, you can easily build your own plugins.
•    You own your data. Piwik being installed on your server, the data is stored in your own database and you can get all the statistics using open APIs (publishing the data in many formats: xml, json, php, csv).
•    The user interface is fully customizable, you can drag and drop the widgets you want to display, create the reports you want.
Similar to Google Analytics, Piwik uses a simple javascript embed to collect data and hence can be hosted independently on a separate server.
Since Piwik is open source, SEO’s can adjust it to fit their company needs. For example, publishing sites may feel that referrals from Digg, Google News and Reddit are more important than the basic search engines and set up their reporting to flag these referral trends first. Bloggers and affiliates may want to view the amount or quality of outgoing links and reference them with referrals and specific keywords, the possibilities are many.