Brain Pop has some free resources. They even have a "free stuff" button to click on.
Have you joined the Homeschool Buyers Co-op yet? In addition to being able to buy curriculum at deeply discounted prices they have links to free curriculum and resources.
National Geographic Kids has games, videos, activities and more. http://kids.nationalgeographic.com/
History for Kids is a site meant for middle school age but I don't think it is beyond some 3rd and 4th graders. Another History site for younger students is Kids Past. They have an online history text book and history games.
The Kids Know It network has links to free resources in all the major subjects along with featured learning activities.
If you need a free kids math site try Cool Math 4 Kids. If you want worksheets then Super Kids will have what you need.
Don't forget Language Arts. Scholastic has free lessons and printables. Random House has reading lists by grade level with accompanying teacher guides. You can even buy the book from them if you want. Glencoe, which is one of my favorites, has excellent reading lists and study guides for the older or more advanced reader.
Hope that gives you a good starting point. Have fun!