I found this resource because I follow @dreambition (Brad Wilson) on Twitter.  Brad has created multiple pages on his website that list many great iPad apps for teaching and learning.  Brad has done a great job of separating all of the apps into certain categories so it is very easy to find what your looking for.  Along with separating many categories, all apps are listed as free or paid apps that are worth a look.  The apps are mostly elementary focused but a lot of the utility and professional learning app are for all students and educators.  I highly encourage you to add @dreambition to your PLN and check his very resourceful iPad app webpage often.