Interesting service. Add one line of code to your template and they offer:
HitTail reveals in real-time the least utilized, most promising keywords hidden in the Long Tail of your natural search results. We present these terms to you as suggestions that when acted on will boost the natural search results of your site. It’s that simple.
Basically, they look at what keywords are driving people your way, analyze the keywords and suggest things for you to write about that will draw more traffic.
It’s free while in beta (no idea how long that will be).