Though $4.99, this app allows you to annotate video. For PE teachers, coaches, music teachers, science teachers or anyone else looking to make notes on physical actions
Though an iPhone app, this would also work on an iPad. Star Walk is a very cool app that essentially is a mobile planetarium. Find visible stars and planets overhead and take the classroom outside.
Designed for 1st-2nd grade students, Murky Reef combines reading and math skills in the context of exploring a coral reef. This is the free version, though there is a more robust version available for $3.99. Students can review numbers, sight words, and vocabulary. There is some reporting built in.
This app allows you to display PowerPoint presentations on your iPad. Could be great for students to use in a setting such as a science fair or history wax museum.