I'm working with a large pilot in Virginia to deploy iPads into Social Studies classrooms. The pilot is called Beyond Textbooks. Below is the current list of apps I'm recommending for students and teachers. Here's an iPad Usability Study, focusing on the usability of iPad apps and websites.

FREE Student Apps

Teacher Apps

ZDNET's Screenshots of 100 iPad Apps here.
Big list of free ipad apps here.
Loads of great tips here.