The concept of the Long Tail describes the phenomenon where the vast majority of statistically less popular items so significantly outweigh the tall head of popular content that there is actually more value to be mined from this "long tail" than the value of the popular items. See the Wikipedia entry for the Long Tail for a much better description.

Anyway, HitTail is a free web service created by Connors Communications (a PR company that has worked with the likes of IBM, Computer Associates, Creative Labs, RealNetworks, Picasa, and tons more) that aims to take advantage of the long tail of search hits which bring readers to your blog by analyzing and suggesting valuable topics that you could write about in the future.

From what I can gather, the idea is that although it's fun to look at the topics that are already bringing lots of visitors to your site, the fact is that you already have content that is satisfying the right search criteria to bring readers on those topics. HitTail watches the odd search terms that happen to bring someone to your site which have the potential to be more valuable to you if you had more content to satisfy that particular search term.

It works like most statistical packages you might add to your site; you insert a tiny bit of code into the body of every page, and it does the rest. You use your account at HitTail to look up all search hits (including the popular ones), popular keywords that are bringing people to your site, and most interestingly a list of suggested topics for future blog posts.

HitTail makes sure to make it clear that they are not a statistical analysis package; they simply want to help you grow your blog by suggesting valuable topics to write about. I haven't been blogging enough lately to see if this truly works, so I'd love it if you could leave your experiences with HitTail in the comments.