Updated post – October 2016.
I wrote about the (free) Desmos Graphing Calculator in June when I first came across it. Since then I have used it a great deal in the classroom as it is a powerful resource, very simple to use and available for students to use at home. Several new features have been added since I wrote that first post. For my school-age students, I particularly like the sliders feature, the trace and the option to use degrees as well as radians for angle measure. You can read the latest blog post from Desmos here.
The ability to share pages is extremely useful for students, particularly now with the addition of sliders; students can be asked to explore families of graphs from simple straight lines for the younger students to polar curves for the Further Mathematicians, perhaps some cubics for GCSE / A Level students. (Note that the value you assign to the variables determines the initial set of values possible with the sliders, click on the values on the slider to edit.) Perhaps explore transformations.
It is very easy to demonstrate a variety of functions, for example, I was recently studying the modulus function with my sixth form students, looking at modulus equations and inequalities a picture speaks a thousand words! (Just type y=abs…)
Thank you Eli Luberoff and team for this amazing resource!