Interactive Math Activities provides an amazing variety of mathematical simulations and visualizations. Note, however, that elementary activities are inter-mixed with secondary activities.

The Kenyon County School District has created a variety of SMART board-based aids for Math instruction which you may find useful. Though most of the aids created are for the elementary sector, middle school teachers may find some material (at the bottom of the web page, useful.

Illuminations - a Java-based K12 math site

The National Library of Virtual Manipulatives - need more be said