Discovery Education has a lot of great resources for educators, many free (over 30). They also have fee-based services such as DE Streaming with thousands of videos, audio files, images, lesson plans, and much more.
The Discovery Education Science Techbook is more than a digital textbook. "It’s a different way of thinking. Thinking outside the box, perhaps. Or more appropriately, thinking outside the book." "The Techbook not only provides awesome content aligned to each state’s standards (there are currently Science Techbooks available for most grades K-8 in 30 states across the U.S.), but includes a model lesson for teachers to use with every concept, assessment questions for each standard, a teacher’s guide, and DVDs for when the Internet is not available. Resources such as the virtual labs, reading passages, video segments, science sleuths, explorations, and interactive glossary, etc., are packaged together to follow the five E model of instruction. Teachers have found an easier and creative way to build lessons using technology resources and content, as well as differentiating their classroom."
They have just announced Techbook 2.0 is available. This is an update to the Techbook based on feedback and suggestions from educators, including a group of well known and well respected educators who reviewed Techbook at a summit Discovery Education hosted. You can Register to attend a webinar to learn about the new layout, features, and enhancements.
25 Free Resources from Discovery Education
Discovery Education’s Science Techbook: Scientific Explanations in Action
What I Want from a Digital Textbook
Apple Announces iBooks2 E-Textbooks - my initial thoughts
What I use with Physics classes instead of textbook
Resources to Replace Textbooks
Free Textbooks - online, download, iPad, Kindle