Google Analytics is an amazingly in-depth, full-featured, and best of all free web application.

In case you're unfamiliar, Google Analytics tracks the number of hits that your website gets. But beyond that, it organizes the data into many meaningful categories that provide a detailed snapshot of who is visiting your site and how they're getting there.

My personal favorite is the map overlay. You can start by seeing how many hits per country your site is getting in a user-friendly interactive map. Clicking on a country brings you in closer - in the case of the United States, you get a state-by-state breakdown of website visits. Clicking on any state then gives you a breakdown by city! Geo-targeting, here we come.

There are a multitude of other features and even some customizable options. It's pretty easy to set up, too; especially with Drupal, which has a downloadable Google Analytics module.

All you need to get started is a (free) Gmail account - try it out at google.com/analytics.