John Page describes his ‘Math Open Reference‘ project as a free interactive textbook on the web, initially covering Geometry.
The tools include various function explorers. Younger students could explore linear functions for example, whilst older students could use the general Graphical Function Explorer to explore any functions, trigonometric for example.
It is very easy to alter the axes and the options including full screen, thick lines and snap to integers make this ideal to display in class.
The sliders make it easy to study transformations and graphs.
There are numerous demonstrations of constructions, for example see this on Bisecting an angle where each step of the construction is clearly demonstrated and explained; once again note the full screen option.
The index gives an idea of the extensive resources available. A site well worth exploring.