Apple's iOS 5 built-in a lot of new, basic functionality previously filled by 3rd party App Store apps, which effectively "Sherlocked" those apps, rendering them redundant in a post-iOS 5 world. Or did it? There's a world of difference between basic functionality that serves the needs of only the most casual user, and advanced functionality with enough control and features to satisfy a hardcore pro.
What Apple did with iOS 5, as with previous generations of iOS, is take away the entry-level business of several prominent 3rd party apps, but still left them with the premium, higher order market. While it's always dangerous creating an enterprise based on glaring functionality holes Apple will almost certainly fill at some point in the future, there's just as much opportunity building a great product that Apple's entry into the space can benefit.
Here are a 5 apps (and a couple things more) I think could make that case.