GraphSight is a convenient tool to plot and explore 2D math functions. You may plot several function in the same window, create several coordinate spaces defined by different variables (not only those y and x), set labels on the plot, use custom user-defined constants in every formula. All of the graphical objects on the plot are much interactive (i.e. they allow dragging, react to mouse clicking and moving). GraphSight gives you a full control over a coordinate space which can be shifted, and zoomed with the only help of mouse.
More info:
hxxp://www.cradlefields.com/gs/graphsight.html