This ed site, which you can be a member or not, has provided a spreadsheet with 100+ must have iPad apps for education and personal use. All subjects are available and is in one place for easy scanning. The list includes free and fee based apps and are available in iTunes.
RH: If you are like me and may be overwhelmed with the many resources available for the iPad. This spreadsheet may simplify your search for apps that you may want to try out for both your class room and personal use. You may have to either go "full screen" or resort to your desk top as you will have to move the slider at the bottom of the page to view the entire spreadsheet.