If you're a blogger, then Lijit is the diagnostic that you thought you needed an advertising budget to run.
Lijit is a widget that allows readers to search your blog content (mine is towards the bottom of the home page). You can see popular terms, as well as the geographic locations that your readers tune in from. Plus, you can link your Lijit to your other online presences (ie. twitter), synching them into one coherent online calling card.