iNaturalist is a web site, and an app, for sharing pictures and observations of plants and animals. Thanks to Richard Byrne at Free Tech for Teachers for posting!
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.
Vernier offers a host of products to support physics instruction, and for $2.99 there is now an app. Students can perform hands-on experiments and collect real-time data through the iPad.
3DTOAD.com is a library of 3D images that you can rotate and explore. It includes skulls, skeletons, dissections, coral, geology, and even music. These could be great visuals on an Interactive White Board or used on an iPad.