Your traffic stats already tell you a lot about your blog. Like where your visitor comes from, which articles are popular, who's sending you the traffic. But every square inch of the screen is prime real estate. Do you know where is the best place to put your RSS feed or newsletter subscription? Your Ads?
And where should the links to your products be placed? Which place on the page are peoples attention drawn to, and where do they end up clicking?
Fortunately, there is an amazing solution available (for free) from CrazyEgg.com. Installing this tracking system will display a "heat map" of visitor behavior, including where they concentrate most on the page, and which part they mostly click on.
I find heat maps especially useful when moving things around or changing the blog theme. This way, you know the new design is effective as well as pleasing to the eye. Beyond just clicks, the nice thing about CrazyEgg is it also tells you which visitors from where are likely to click on which links.
The system can be used for many other kinds of tracking of sales and advertisements. Alternatives to CrazyEgg are also available if you want to compare.
ClickHeat - this is actually an open source script from Labsmedia.com, which you can install on your own site. Works a lot like CrazyEgg, maybe not as detailed reporting.
Feng-GUI.com - uses artificial intelligence to simulate what users will click at based on a picture of your site. So it is an estimate. There is a Firefox plugin called "View Finder website heat map" to make using this tool easier. Might be useful when you start a new theme but I'd definitely follow up with a real click tracking tool like CrazyEgg.
Clickdensity.com - much like CrazyEgg but looks like it has A-B testing in the premium product that CrazyEgg doesn't.