A tool that is so powerful and free is always going to be an essential in the zero budget marketing tool kit. I'm talking about Google Analytics.
Yet Google Analytics provides you with so much information about your business that you often only take in the basics - how many visitors, how long are they spending, what page are they leaving from, what is the bounce rate (where they only visit one page and leave) and maybe how many goal conversions or what key words are bringing people to your site.
It is worth digging deeper occasionally, as it may reveal some business issues that need your attention. This can be less about analysis and more about recognising a trend.
Here's one example...
Browsers - with many of the clients I help manage, I've been watching an interesting trend emerge over the past couple of years. By far the most popular browser was Internet Explorer...but no longer. Gradually Firefox gained traction and interestingly, Safari is also now extremely popular.
Why the Safari dominance? The proliferation of iphones and ipads.
Why does browser matter? If your website was built more than 3 years ago, it may not have been tested well (or at all) on a browser like Safari. So does it work as it should on Firefox, Safari or Chrome? And if not, how much of your hard won traffic is getting a sub-standard experience?