Brain POP is not free, but they are so confident that you will love their content, that they offer a free 5 day trial period for you to explore the sites on your own. It is a little pricey ($80/yr) but they do offer a monthly payment plan ($7.95) if you choose the "Family" subscription plan instead of the "Homeschool" plan. (It is a $1 extra for BrainPop Español.) I think that if you would actually use it on a weekly, if not daily, basis, then the price may be worth it. If not, be sure to at least stop by and look at the free videos and stuff that they offer.
Here's a breakdown of the sites:
Brain POP ESL is good for those of you beginning to teach your students/kids English as a second langage. (Man, I wish they had a site for those learning Spanish as a second language!)
Con mucho cariño...