Annenberg has a ton of educational videos that they sell for different grade levels. Their website has a ton of great supplemental tools, including interactives. The disciplines included are Arts, Foreign Language, Literature and Language Arts, Mathematics, Science and Social Studies/History. Check out other parts of the site for more lesson plans and video clips.
National Library of Virtual Manipulatives
Most of the math interactives on this site are java applets. The selection is small than some of the other sites here, but there are some useful tools to explore here.
There are just a few tools on this page, but they are gold for writing and research. One tool helps you build a thesis for a persuasive essay. Another suggest topics for papers. A third helps students build a structure for a cause and effect essay.
Thinkfinity is online community created by the Verizon foundation. the site's content partners are a number of discipline specific portals that have lesson plans, interactives, multimedia and links to other resources. Direct links to the individual content partners are below.
Econedlink (Economics and Personal Finance)
National Geographic (Geograpjy)
Read Write Think (English)
Smithsonian's History Explorer (American History)
Utah Education Network
The Utah Education Network has a number of interactive activities for a range of grades and disciplines.