Between the Lions Well-known Video resources are among the best!
Age Range: Pre-K - Grade 1
ReadWriteThink Student Interactives, from the International Reading Association and National Council of Teachers of English, are jam-packed with lessons and interactive games for students of all ages learning reading and writing skills.
Age Range: K to 2nd grade
OxfordOWL is another site from the UK. It offers 100 free ebooks that range from simple level-reader stories to more complex titles and layouts, such as "How to Build a Castle".
Age Range: Pre-K to 5th grade