This applet teaches students in a very dramatic way how changing the "bin width" of a histogram can dramatically alter the shape of the histogram. Lesson: you should graph your data with a number of different bin widths and choose the one that you feel best represents the data.

http://www.stat.sc.edu/~west/javahtml/Histogram.html

Here are two versions of Completing the Square - a web page from NCTM and an ActivStudio flipchart.

Completing the Square web page
http://illuminations.nctm.org/ActivityDetail.aspx?ID=132

Completing the Square flipchart

An on-line slide rule. Brings back memories....
http://www.syssrc.com/html/museum/html/sims/javaslide/

Click on the title above to download the slide rule in an Activstudio flipchart.

Move the red dot on the parabola and the gradient function (a linear function) is graphed below it. Very neat. From Manipula Maths.
http://www.ies.co.jp/math/java/calc/x_diff/x_diff.html

This applet also draws the graph of the 2nd derivative.
http://www.ies.co.jp/math/java/calc/x_2nd/x_2nd.html

Move the red dot on the trig function and the graph of the gradient function is shown below. Very neat! From Manipula Maths.
http://www.ies.co.jp/math/java/calc/sin_diff/sin_diff.html

This applet also shows the graph of the 2nd derivative.
http://www.ies.co.jp/math/java/calc/sin_2nd/sin_2nd.html

A man is riding on the surf. We set f(x) as the curve of the wave . Observe the slope of the surfbord. The trace of the slope is the derivative of f(x). From Manipula Maths.

http://www.ies.co.jp/math/java/calc/doukan/doukan.html

Changing the Border
This is a nifty puzzle whose solution is based on the area of a triangle. The land is divided into two parts (P and Q) by the border ABC. Redraw a straight border line passing through point A so that each of the areas does not change.
http://www.ies.co.jp/math/java/geo/tochia/tochia.html

Dividing a Quadrilateral
This is a more challenging problem that is based on the same underlying concept.
http://www.ies.co.jp/math/java/geo/shi2to/shi2to.html

Some wonderful animations that show the sine, cosine and tangent of angles on the unit circle.

Sine Box
http://www.ies.co.jp/math/java/trig/sinBox/sinBox.html

Cosine Box
http://www.ies.co.jp/math/java/trig/cosbox/cosbox.html

Tangent Box
http://www.ies.co.jp/math/java/trig/tanbox/tanbox.html

This is a neat look-see proof that the sum of the exterior angles of any polygon is 360 degrees.
http://www.ies.co.jp/math/java/geo/gaikaku/gaikaku.html

Move the triangle by dragging the red point. What did you find?
http://www.ies.co.jp/math/java/geo/san180/san180.html