The district wide iPad committee that I served on was tasked with creating a list of core apps for the iPads. There was significant thought and care put into making the list. We wanted to stick with free apps, developmentally appropriate, focused on creation yet considering space limitations on the device.
We liked this visual for sharing the apps so we tried to recreate our list in this style. http://www.teachthought.com/apps-2/47-core-teacher-apps-a-visual-library-of-apps-for-teachers/
This is the list as it stands today for elementary, both teacher and student versions.