So far all 7th grade Humanities classes use the following apps the most and seem the most relevant to 4/5 implementation. Currently our iPads are used mostly for resource websites and readings/ research via our first class web page.  We guide them toward selected reading or viewing material-    I am a "newbie" to iPads, but this is what I have learned to love and use most often...my list however might be different come May!  I tried to keep this list to apps that would work beyond social studies, and please keep in mind I have a HUGE learning curve- just starting out on this adventure and all!
1.  Pages
2.  First Class (free, and all kids have a first class email at RMS) we use communities and kids drop summatives...email me questions, and working on how to effectively use "Chat"..plenty of opportunity here.  Takes managment.  One
3.  Revealer on desktop and mirroring (technically not an app, but the only way to show my iPad on active board, and students can "reveal" their work on the active board as well)  I use it for group work sharing, tickets out the door, etc...  The kids love it, and so do I!  Best ever for sharing and talking about work
4.  Nat. Geographic Atlas and mag app
5.  ABC News app
6.  CNN app, but CNN student news does not have it's own app:(
7.  Show me (alternative is write it)- used most with reveler- kids use it to make Khan like videos:)
8.  Brain Pop- we use school account so kids log in easily
9.  Evernote...still working kinks out since 5 students share 1 iPad- but the potential is beyond GREAT
10.  Khan academy
11.  Mind Map- currently we use this to make Thinking Maps, since Thinking Maps does not have an app..we are using free version, but worth it to pay for upgrade or get Thinking Map folks to build an app!

One another note- The jury is still out for me regarding My Big Campus. At first I thought this was great and we used it a bit...every kid has an account, but it has many limitations recently.  I was excited for such a tool to share relevant YouTube videos and other videos that people use YouTube as a host.   Mostly the limitation is because every YouTube video now comes with a commercial, and any video with a commercial is blocked on our CSD server...so from school when you go to watch it says "error occurred".    I am able to bundle videos on topics for kids, kids can follow me like facebook, and some tools existed for creating quizzes etc...  This site  has been frustrating.   I have to spend time at home creating the bundles which work, but then I get to school and the dreaded "Error occurred" takes place.  Many other teachers for some reason also mention My Big Campus with frustration. For social studies providing videos and clips is important way to get info to kids in a differentiated way.  Many reinforce, teach, or enhance what they are learning!  I  admit I may need more training about My Big Campus, but am wondering what else is out there to meet this need.  I have discovered SQUIRREL, and it's cool, but I can't add my own videos...so maybe another expert is needed in the field of best way to access and download videos:)

Found this list of social studies apps:
http://wss.sd73.bc.ca/wiki/index.php/Best_Social_Studies_Apps_for_iPad