If you aren't doing any sort of analytics for your website, you should be. Analytics is useful because it tells you what people are consuming on your site, and how they are consuming it. In theory, most web pages are designed to be consumed by visitors, and this means that you need to understand your visitors in order to build a quality site that meets their needs. Understanding your analtyics and your users should also have the goal of growing your site traffic by responding to the things you learn.
There are two great tools for tracking usage of your site. The first one is called AWStats and is an open source project. The second is called Google Analytics, and is a proprietary service provided by Google. I recommend using the combination of these two tools to get a more complete picture of your site.
Use BetterAwstats as a drop-in front-end replacement for Awstats. This tool is great for managing multiple sites (data can be combined, or browsed more easily through the menu on the left). It is also great because it allows you to more easily see long-term trends such as multi-year month-by-month changes.