This is an excellent introduction to the vertex form of a quadratic function.

http://www.heymath.com/main/samples/us21/teacherstemplate.html

This web page isn't particularly interactive, but it is an excellent application of quadratic functions.

http://www.heymath.com/main/samples/us20/teacherstemplate.html

This is an excellent introduction to the topic of simultaneous equations.

http://www.heymath.com/main/samples/us19/teacherstemplate.html

This web page does a great job of introducing the balance method to solve equations.

http://www.heymath.com/main/samples/us18/teacherstemplate.html

This is an interactive one quadrant number plane
http://www.teacherled.com/resources/onequadgraph/onequadload.html

And this is an interactive four quadrant number plane
http://www.teacherled.com/resources/fourquadgraph/fourquadload.html

A fantastic implementation of the 'bath graph', ie the graph of water height vs time if a bath tap is turned on/off, and/or the plug is pulled out, and/or a person gets into/out of the bath. And a yellow duckie is included for free. Three thumbs up!

http://www.mathsonline.co.uk/nonmembers/gamesroom/sims/archi/archi4.html

A Java applet in which students can change the number of points and the correlation coefficients, as well as add points. They can also show the regression line and the residual plot. Three thumbs up!
http://stat-www.berkeley.edu/~stark/Java/Html/Correlation.htm

An interactive lesson by HippoCampus on variables, expressions and equations.
http://tinyurl.com/5zwjvc

All of the HippoCampus interactive lessons can be found at
http://www.hippocampus.org/

Six points are shown along with a line that passes near them. The applet shows the square of the residual of each point as well as the sum of the squares. Your task is to change the gradient and y-intercept of the line to minimise the sum of the squares. Three thumbs up!

http://www.dynamicgeometry.com/JavaSketchpad/Gallery/Other_Explorations_...

Add some points to the scatterplot. The applet will calculate the correlation coefficient and the sum of squares. You can also choose to show the least-squares regression line and the x-mean and y-mean lines. Very instructive!

http://bcs.whfreeman.com/yates2e/pages/bcs-main.asp?v=category&s=00020&n...