Piwik is an open source analytics program for viewing your web site traffic and traffic patterns. This is useful to help you sculpt your site into what really works for your visitors. You can study a lot of metrics and learn what pages are popular and which ones are not. For example, you can see what pages get a high bounce rate which means that your visitors take one look and hit the back button. Piwik can also help you track down 404 errors and other details that add up over time for your overall traffic.
Stats You kind of get to wondering what is happening with your sites...
We have recently moved a number of our sites and client sites to Piwik. We like the over simplicity of the GUI and the fact that you can embed your stats on your web site or blog if you want to. Plus the program and the API are open source. We like that we can track the stats for a number of web sites in one simple to use interface. We made use of the feature of using special tracking code our new 404 error page, blocking our own IP so we don't to show up in our own stats and learned how all from the easy to follow FAQ. We Like it, we use it and we recommend it!
According to their web site: Piwik is a downloadable, open source (GPL licensed) real time web analytics software program. It provides you with detailed reports on your website visitors: the search engines and keywords they used, the language they speak, your popular pages… and so much more.