Khan Academy has a vast number of lessons across curriculum areas, for kindergarten through to the 8th grade. These lessons are excellent for students to use at home but I incorporated them into my Blended Learning Classroom.
Starfall is a fantastic resource for younger students or English Language Learners (ELL) students. Within the Blended Learning Classroom my ELL students were able to develop their English phonic skills through colorful and easy to use lessons.
Some other great online resources for mathematics in the Elementary grades are:
This lesson is a lesson on squares and rectangles and demonstrates how this could take place as a flipped lesson.
This lesson is for teaching Digitally Literacy to a 4th grade class.