Public, private, home school, and “just for fun” teachers will appreciate the free resources available through the Discovery Education web site. Start at their home page: http://www.discoveryeducation.com/ and scroll all the way down to the bottom to see the links.
As with any free tool, you’ll have to put up with a little bit of advertising, but the sites are fairly well organized and easy to use. I didn’t run across any tools that required the user to set up an account. What a relief, in this market-driven world!
One of my favorite links is Learning Adventures under Free Teacher Resources.