What Are the Must-Have Classroom iPad Apps? Please Share Your List

In my previous post, I included an excellent video that I think captures very effectively how the iPad can transform the classroom into a 21st century learning environment. Larry Mitchell's students are the picture of high engagement as they tackle both normal classroom tasks and authentic learning tasks.

The simple truth regarding any technological device like the iPad is that it's only as good as the applications it is using. My next question is, "What are the best iPad apps for the classroom?" I have written several times regarding those apps that I think administrators should use, but what would be apps to include on every iPad we put in classrooms?  Feel free to share your thoughts below. I hope to share out responses at a later date.