3D and Contour Grapher
You can use the following Flah applet to draw a 3D graph. You can toggle between 3D Grapher mode and contour mode using the large button at the bottom.
In Contour Mode, you are looking at the 3D graph from above and the colored lines represent equal heights (it's just like a contour map in geography). The green lines are lowest and the red ones are highest.
When you get into 3D Grapher mode, enter a function of x and y or explore the demo ones available as links on the bottom right of the 3D Grapher screen.
If you mouseover the graph, it indicates x- and y- coordinates.
NOTE: This applet is Flash-based, so won't work on your mobile device.
Flash applet by FlashandMath.