Serps.com. It’s a pretty sweet tool that tracks your rankings and gives you personalized reports that you can customize for different clients. I love their reporting functionality. Ever since Raven stopped showing some of the data I have relied on Serps.com.
I think especially for agencies, you can CNAME the app to your own domain, so dashboard.highervisibility.com<http://dashboard.highervisibility.com/> for example, and fully white-label the app in just a few minutes. Makes it look like I’ve got some sweet custom software for them (at no extra charge)
Something else that really sets it apart from everyone else is their alerts function. If they detect a higher than normal amount of rankings shift for my site, they send me a detailed alert with everything I need to know about it. It’s awesome so you’re not caught off guard. It’s little things like that, that give me a competitive edge above my competitors.