Khan Academy:
Lessons in math as well as reading and writing skills at all elementary levels
PBS Learning Media
https://gpb.pbslearningmedia.org/
Compilation of videos, interactive lessons and lesson plans for a variety of subjects (including science and social studies) at different grade levels.
Free resource. Requires no sign-in.
Read, Write, Think
http://www.readwritethink.org/
Lessons plans and interactive activities for all grade levels on everything related to reading and writing.
There is an “afterschool” section for activities to augment what students are learning in class.
Cool Math for Kids
https://www.coolmath4kids.com/
Math Games for kids ages 3-12.
Starfall
Language arts and math for children K-3rd grade.