The site includes lots of free activities inspired by Montessori and constructionism. Most of them are geometry activities for elementary students, but there are also online guides to help kids use Logo to explore areas like algebra and precalculus (I use Philip Lewis's text in an online course on precalculus & Logo with gifted kids in grades 6-12).
There's a free downloadable software package called Circular Reasoning, which I'll be speaking about at the NCTM conference in Denver on Saturday, November 12. The talk is "Exploring Angles with MicroWorlds and Circular Reasoning".
The site also has activity cards for the Circular Reasoning software, and video and articles with ideas on teaching, curriculum and software design, and research.