If you have a website, Google Analytics gives you a lot of useful website stats for free. You can see how many visitors you have had, how long they were on your site, how many pages they viewed, how they got to your site, what browser they used, where they are from, etc.
There are several iPhone apps that let you access that information on your phone. I tried out several of the apps in this category, but Analytics App was superior in my opinion. In addition, support is knowledgeable and responsive—always an important consideration.