Time4Learning.com - You do have to pay for this site. While I have not personally used it, I've heard great things about it from many homeschoolers. Covers Language Arts & Math for pre K - middle school, and Science & Social Studies for grades 1 - 7.

 

StudyBeat.com - This used to be Studio 4 Learning. It appears to still have the same material (Standard Deviant videos on a variety of subjects), but now charges a small monthly subscription fee.

 

BrainPop - This site has videos on many topics & subjects, with quizzes & activities. There is also a BrainPop Jr for K-3 and a Spanish BrainPop. It appears that you can only do annual subscriptions, not monthly. They do offer a free trial, so you can try before you buy.

 

Discovery Education Streaming - A collection of thousands of videos, with teacher's guides, quizzes and more. There is a subscription option for homeschoolers.