Flexible, programmable graphic applet makes graphing intuitively hands-on.
GeoGebra is a great, powerful application for demonstration and for student investigation. I use this for students when a graphing calculator is too small for students to see well, and when I want students to manipulate values with sliders. I love that I can tell students it is free to install at home.
How I Use It
I use GeoGebra when I need to quickly demonstrate how changing values in algebraic statements affect the graph. This is great when demonstrating slope as a rate of change. Students can use this on laptops in the classroom or on the interactive white board as a demonstration. It's great for helping students understand why a vertical line has an undefined slope.
I also use this with geometric figures, so help students grasp abstract concepts concretely. I have used it to visually demonstrate the formula for area of a circle, and to illustrate rigid transformations of polygons.