A great way for students to use the itouches in the library is use of the NASA application. My students each chose a planet to do a slideshow for. They conducted research on computers and could have used the NASA application for additional information.
Another cool application is the United States Geography by Discovery Education. My students are asked to compare and contrast different places in the U.S. (we, as a class, analyzed Alaska and Arizona, and each students did a Venn Diagram on the kidspiration program). This application could help students (most of mine have lived in Texas all their life) understand different types of geographical features.
There is a really cool application titled 101 + 10 new science experiments. It cost 3.99 so I did not download it but the demo lists a few of these experiments and they give one example of a detailed science experiment.
There are many different applications for students acquiring a new language (whether it be English, Spanish, French, American Sign Language, etc. This might be helpful in the bilingual classes as well as for ESL students (for that matter, any student could benefit from improved language and grammar skills).
Art Lite is a great art application for those interested in seeing great works of art (for are differentiated learners who love to use artistic skills to demonstrate understanding, this would be a fun app. explore. I would be careful with this one as some art can be suggestive of adult themes.
There are so many great apps. that they are too numerous to list. I think this is a great tool for differentiated learning and I am envious to today's students that have so much more information available to them than when I was their age.