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
National Library of Virtual Manipulatives - need
more be said