This article from I Education Apps Review lists out a series of apps to use for creating custom digital content. Not all are free, but the list presents some interesting options.
Students can create Trading Cards based on people, places, events...I haven't played around with it yet. Wondering about the ability to export content / share content from the app.
This is an interesting classroom management concept for 1:1 situations when teachers want to control the presented content. Not sure about the ability for students to interact with content, but they could take screencaptures and add to their notes easily. Lots of potential.
This is a great list of ways to curate content. Most of them also have apps. Diigo and PearlTrees work well on iPad. Flipboard is another great option.
This claims to be the first browser created by a doctor that is just for kids. The idea is that parents and educators curate content and students then can only access what they have identified. An interesting concept to present curated vs. filtered content.
This is an interesting program that combines student response system, content delivery system, and student monitoring all in one. Schools can apply for the new iPad pilot program for free (at the moment).