Note that all the material on this site has been incorporated into Mathematics for Students.
All further updates will be made on that site.
From Desmos comes an outstanding graphing calculator which allows you to easily plot one or more graphs, including lines of the form x=k. It is also possible to illustrate inequalities.
See this page for more information.
Also from Mathisfun you can explore the properties of straight line graphs and plot more complex functions using the Equation Grapher which you could use to plot a circle for example. (For example x2+y2=9 is a circle, radius 3, centre the origin)
GraphSketch allows you to plot functions and get a permanent url to your graph, see this example.
Settings are easily changed. Scroll down the page for information on how to enter functions.
For the modulus function use f(x) = abs(x).
Graphing Calculator from Holt Online Learning.
Enter up to 4 functions, note that you can use the blue keys on the right as well as the keyboard.
Use ^ for powers, eg x^3 for x3.
Press Graph to graph the functions. Use the Settings tab to change the axes.
To find the coordinates of points of intersection of two graphs, use the Intersection tab, select the graphs you want and then choose Find Intersection Point(s).
If you are plotting trigonometric functions note that you can choose degrees or radians.
This is a useful tool for studying transformations of graphs; for example look at the graphs of sin(x) and sin(2x) on the same diagram.
Explore quadratic curves with this tool from Math Open Reference.