Though this is not necessarily a post specific to the iPad, many of the suggested resources either have apps or can be used on the iPad via a web browser. There are some interesting concepts for ELL/ESL or even foreign language learning.
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 a great tool for sharing files. Drag the file into the browser and Pastelink creates a URL to share. This works for files up to 250 MB and does not require a login or email address.
Not only does BaiBoard allow students to collaborate on a shared whiteboard via their iPads, but they can also jointly annotate PDFs. Much like with AirSketch, BaiBoard also includes the ability to do a web share. This means that when on the same network, and with the unique URL associated with the iPad, ANY device can see a student working on their BaiBoard through a browser.