GeoGebra is dynamic mathematics software for all levels of education that joins arithmetic, geometry, algebra and calculus. It offers multiple representations of objects in its graphics, algebra, and spreadsheet views that are all dynamically linked. WE LOVE GEOGEBRA IN 7TH GRADE!!
Download GeoGebra on your computer http://www.geogebra.org/cms/
There are many different websites that offer applets available to use and learn from.