Great FREE reading app for pre-school/early readers. Students work through puzzles and games for each letter of the alphabet and build their vocabulary.
With Pocket, it's possible to curate content to read offline or online. It has similar features as Diigo, but uses a visual interface similar to Flipboard. Definite potential here.
Another wiki page that is under construction, but a great source for finding social studies apps. The page is broken down to address maps, social studies, references, and current events.
This app that allows you to split the screen on your iPad so that you can view multiple apps at one time - much like on a computer. It also has Dropbox support, and a Pro version is available.