ReadWriteThink is a free collection of lesson plans, student resources, and professional development resources. The site is a project of the International Reading Association and the National Council of Teachers of English and is focused on reading and language arts, but the site also includes a number of interdisciplinary resources and tools for digital literacy and learning.
The International Reading Association has also produced several free iPad apps that could be used in a number of subjects, including Timeline, Venn Diagram, and Trading Cards.