I will admit to having a mild fetish for Google Analytics. Yes, I wish it updated more often as I would gladly stare at a real-time traffic analysis for this blog, but perhaps it's healthier this way....
At any rate, Piwik (formerly Phpmyvisites.com) looks to be a reasonable (and open-source) alternative to Google Analytics. You can watch a demo of it here.
Who cares if it's open source? Well, perhaps you should:
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 limitless.
Amen. The power to tweak. The right to fork. That's open source.