The Art Exchange

http://www.ccproject.org/artexchange.html

Creative Classroom Projects, a part of the nonprofit Creative Connections, hosts this international art exchange. The Creative Connections Project is a sponsored project of The New York Foundation for the Arts, a non-profit organization, and some of their resources require the payment of a small registration fee. However, there are sample free teacher resources, and there are extensive student art examples from several corners of the world. A great idea!

Interactive Mathematics

http://matti.usu.edu/nlvm/nav/vlibrary.html

From Utah State University, with funding from the National Science Foundation, comes the National Library of  Virtual Manipulatives for Interactive Mathematics. Tied to the National Council of Teachers of Mathematics standards, and complete with lesson plans and helps, the site's virtual manipulatives cover a variety of subjects on all grade levels. These tools are low-tech by web standards, requiring nothing more than Java in almost any current browser. Great online tool!

Body and Mind

http://www.bam.gov/

Sponsored by the U.S. Government's Center for Disease Control, BAM is a site full of activities for kids on the subjects of mental and physical health, as well as illness and nutrition. There's a section with some limited teacher resources, with links to other education-related health sites.

The Literacy Center

http://www.literacycenter.net/

This free website service was originally funded by a U.S. Department of Education Small Business Innovation grant. It is dedicated to providing simple, free learning tools for young learners (it's appropriate for K-2) with a focus on early ESL. All games and activities are Flash driven.

The U.S. EPA's Air Now

http://www.epa.gov/airnow/

The U.S. Environmental Protection Agency provides this visual database of U.S. air quality, including teacher materials and instructional ideas (look for "Environmental Education" in the "For Teachers" section). There's even some online activities for elementary school kids.

Congress for Kids

http://www.congressforkids.net/

This project from the Dirksen Congressional Center in Illinois has information pages on the history of the U.S. Constitution, the branches of federal government, the electoral process... and all with regular formative quizzes along the way to encourage learning. It looks like it's aimed at middle to early high school. The content is high quality and well laid out, but it's primarily text based, and I couldn't find any teacher's resources.

MidLink

http://www.ncsu.edu/midlink/

North Carolina State University, with support from SASInSchool, and the University of Central Florida, bring this online literary magazine for students from 8-18. There are instructions on how student writing can be submitted, an archive of past editions, and a teacher resource section with lots of links to sites in support of writing.