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 is the app version of the Mac program and offers a comprehensive solution for diagraming, charting, designing pages, creating wireframes, and more. It is expensive, $49.99, but full of features.
Though $7.99 seems steep for an app, not only does this one allow you to edit Office documents (same as Pages, Keynote, & Numbers), but it also lets you work on Google Docs.
This team of teachers from the National Teacher's Academy did an excellent presentation at the EdTechTeacher iPad Summit. This site lists some of their favorite tools, apps, and sites.