Thousands of free audiobooks in the public domain. Readers can download also download text versions of the books onto readers. Many different genres and age levels available.
Free literary and reference materials. Over 30,000 titles available. It was mentioned in "Integrating Technology and Digital Media in the Classroom" (Shelly, Gunter, Gunter 2010), but I thought it was important enough to make another note of.
This is a website that is great for providing your students with extra support on problematic areas in the classroom. I think this website is best for worksheets. They offer ebooks and they are organized by topic. Membership for this site is free; however, their suggested sites (i.e. vocabularyatoz and writing atoz) are not free. Overall, this site is great for content specific extra support in the classroom, in all core subject areas.