National Council for Teachers of Mathematics
The site of the National Council for Teachers of Mathematics (NCTM) has resources, lessons, and activities for elementary, middle school and high school math teachers, as well as the national mathematics standards and professional development information.

Illuminations
This site is a collaborative effort between the NCTM and the Marcopolo Educational Foundation. It has pre-K through grade 12 lesson plans, Web resources, interactive math tools (Java applets that you can use to explore mathematics), i-Math interactive multimedia investigations, and video clips about teaching practices for mathematics.

High Performance System
The High Performance System (HPS) web site allows you to download a demo of STELLA for either Mac or PC. This site also provides an opportunity to discuss the software with other members of the HPS community.

HyperStudio
Hyperstudio may be downloaded from this site (for a cost), but the really interesting part of the website is the links section, which takes you to other HyperStudio-related websites, such as sites where you can share math stacks and earth science stacks.

Center for Highly Interactive Computing in Education
The Center for Highly Interactive Computing in Education, which is focused on science and mathematics software for the handhelds (Palm and others), has free software specifically designed for middle school and high school students to use. There is also access to projects to do in middle and high school, and to teacher professional development.

Creative Learning Exchange
The Creative Learning Exchange provides access to many learner-centered models and lesson plans based on STELLA or similar system dynamics software. It is also a place where teachers can discuss in a forum and list/share links to other sites of similar nature.