Regarding features Piwik doesn’t leave much to desire: visitor graphs (beautiful!), detailed information on the user’s origin (search engine incl. search terms, website url), browser setups, the visit’s duration and provider. Even entry and exit pages, as well as automated reports via email and the option to add more functionality through widgets are included. It’s also possible to anonymize your visitor’s IP addresses to satisfy country specific privacy protection laws.
For WordPress users the plugin wp piwik (which is not developed by the same group of people) is also worth mentioning as it integrates smoothly into the WordPress backend by adding graphs and statistics to the dashboard.
What impresses me most, is that this multitude of features doesn’t make the interface clunky or difficult to use – which seems to be a common problem, especially with big open source projects. Again it’s just nice to find an open source project which is easy to install and meets or exceeds the requirements!