ABCya is a brightly colored site similar to Starfall, covering Pre-K to 5th grade. Learning activities are free and often center around educational games.
Fun Brain has games, video, and books for K through 8th grade students
Highlights Kids has games, crafts, recipes and jokes for preteens.
Into the Book has reading activities and strategies in English and Spanish
Khan Academy was founded by Salman Khan, an entrepeneur and educator. Khan Academy focuses on STEM courses for K through 8th grade, though many subjects are college quality. Subjects include maths from Arithmatic through Linear Algebra, Physics, and Computer Science. There are also Arts and Humanities courses, and test preparation courses for the SAT and many others. Arguably, this is the highest quality and in depth free resource for young students, and is popular with home schooling families.
Multiplication.com teaches maths through games, flash cards, videos and quizzes. It covers 1st grade through High School. Memberships for Teachers, Parents and Students are free for online activities. Physical Books, eBooks and downloadable flashcards are available for purchase.
National Geographic Kids teaches geography, animals, and culture
PBS Kids public broadcasting website mirroring PBS education channels (auto-plays audio)
Scratch teaches programming to kids! They can create stories and games, then share with their friends.
Starfall is for Pre-K and young students can learn in an engaging environment that covers the fundamentals: ABCs, Reading, and Basic maths through games, songs, and videos. Parts of the site are free, full use is covered by a $35 fee that grants access for a year.
Storyline Online is a collection of several dozen book videos showing illustrations, and read aloud by famous actors such as Lily Tomlin and James Earl Jones.
Switch Zoo teaches about animals and their habitats.
Worksheet Fun has dozens (hundreds?) of grade school worksheets covering major subjects, a trove of PDFs for youth education.