Grades PreK-6: National Geographic offers this free and engaging resource for students that would like to dig a little deeper! National Geographic developed the popular Weird But True! series. Well, there are many Weird But True! videos and articles available here! National Geographic is particularly good for learning about animals, US states, space, history, and general science.
Grades K-12: This is an easy-to-use research and reference tool that provides accurate and reliable content on almost every core subject. Take time to explore the world and US atlases, use the search bar to find your research topic, and take note of the freely given reference information at the bottom of each article!
Grades 6-8:The Library of Congress (LOC) is the largest library in the world, with millions of books, films and video, audio recordings, photographs, newspapers, maps and manuscripts in its collections (Carla Hayden, 14th Librarian of Congress). This library is the research library for the US Congress and the general collection is available to everyone (online or in Washington D.C.).
Two links are useful here:
Grades K-12: Reference work and research is synonymous with Britannica! It's a classic. Britannica School is separated by Elementary, Middle, & High School to best serve student interests and information needs. The state of California has purchased a subscription to Britannica School for every student in the state. It's that good, and you'll find it all here.
Grades K-8: TeachingBooks is an excellent resource for both teachers and students. Do you want help finding your next book? Have you ever wanted to meet-the-author? Would you like to see trailers for books before you commit, or listen to the authors read their own work? Explore this website! It is free for all California students and teachers.
Grades 7 & 8: You will find the world’s great "Classic Literature" here, primarily older works for which U.S. copyright has expired (think Frankenstein, Little House on the Prairie, Pride and Prejudice, etc.). Thousands of volunteers have digitized and proofread the eBooks that are available on Project Gutenberg. Books are available for download or online reading.