Ask an astronomer is run by volunteers in the Astronomy department at Cornell University. It's a free resource that includes an abundance of weird and wonderful details, facts, stories and images on all things astronomical.
Scholastic's Choices Magazine is written for a teen audience and includes informative and entertaining articles on life skills and health.
E.O Wilson's Life on Earth is a series of free rich-media Biology textbooks in iBook format that can be read on their own, or studied in partnership with a free iTunesU course.
The Explore section of the National Museums Scotland site has some great stories, videos, activities and resources inspired by the museum's collection.