Slope of a Line
The slope of a straight line, parallel and perpendicular lines are all explored interactively using an applet.
There are two applet one in HTML5 and the second one in Java. You may use any of the two to explore interactively the concept of slope.
Interactive Tutorial Using HTML5 applet
Interactive Tutorial Using Java Applet
Slopes of Parallel lines
 On the left panel, click on the check box "two
lines". Two lines are now displayed and you can change their position by
dragging points A, B, C and D. Change the position of one line so that the two
are parallel ( no point(s) of intersection). Compare the slopes of the lines.
Slopes of Perpendicular lines
 Change the position of the lines so that they are
perpendicular ( make an angle of 90 degrees). To make sure that they are
perpendicular, adjust the lines so that they go though the diagonals of the
small squares in the rectangular system. Find the product of the two slopes. Do
this last experiment several times. What can you say about the product of the
slopes of two perpendicular lines?
More references on slopes.

