Google Analytics

Google Analytics a free tool from Google that monitor your site traffic. It shows you how people found your site, how they explored it, and how you can enhance their visitor experience.

With over 80 reports, Google Analytics account can track visitors through your site, and keep track of the performance of marketing campaigns - like AdWords campaigns, email campaigns, or any other advertising program. With this information, you'll know which keywords are really working, which ad text is the most effective, and where your visitors are dropping off during the conversion process.

Even if you do not represent a commercial entity...you will find it really amazing...& contains real deep details about your traffic...data fetched by analytics includes a lot of details like:
users languages, network locations, browsers, operating systems, screen colors, screen resolutions, java support, Flash version...etc

For more info...

take a Google Analytics tour:
http://services.google.com/analytics/tour/index_en-US.html

check Google Analytics blog:
http://analytics.blogspot.com/

not to mention Google Analytics itself ;)
http://analytics.google.com/