A few months ago, I subscribed to the HitTail.com service. I then integrated it to this site with the purpose of testing it and learning more about it, as well as optimizing my site.
I must admit, their system is pretty interesting. Described in few words, it is a very clever arbitrage for the PPC and SEO market. The idea is based on The Long Tail book by Chris Anderson.
Once integrated to your site, the system analyzes the web searches brining traffic and calculates the frequencies of the different keywords and their combinations. It then presents you with your tail shape, showing you the most popular (top ten) keywords and also the long tail (keywords coming after your top ten), which are not so popular, but that (they claim) are responsible for a much bigger ammount of your traffic resulting from web searches.
For example, for this site, some the most popular keywords / combinations are: e1705, linux, warren buffet, dell inspiron e1705, ubuntu, kubuntu, etc. My top-ten keywords are responsible for brining 20% of my total search-result-induced traffic, while my long-tail keywords are responsible for the other 80%. This probes their point, suggesting that working on the long tail keywords to induce natural searches can dramatically improve your overall SEO strategy. Also, working on your long-tail keywords can help you reduce your advertising expenses, since by getting more natural search results induced traffic, you wont necessarily need to force PPC-generated traffic as much (altough, of course, you still want to analyze your overall stratege, maximizing your margin and leveraging from your liquid assets as much as you can).
After presenting you with this summary (and also with a live-view of the keywords referring traffic to your site), the system can give you suggestions. Again, as an example, for this site, that's something like: family guy stories, schlumberger work blog, guy sebastians address, emoticons da family guy, software engineer gemalto austin, dell inspiron e1705, sebastian com, las ruinas circulares de borges, etc). As you can see, the suggestions are related to my previous articles. [note: using their suggestions turned out to be pretty easy, right? ... actually, you should use them in your titles, in bold and big fonts, etc].
Some of the suggestions (as you can notice above) are not really interesting (remember this is computer-generated), so you also have a way to move the interesting ones to your to-do list and check them once you have used them on your site.
If you would like to use this system for your site but don't really know how to set it up, I would be more than glad to do that work for you for a reasonable fee. Please contact me.
Also, there is a youtube video that explains the idea in a better media: