Even though there are many toon shaders out there,
you may be in a situation where using one just isn't possible, and you
need a quick fake.
First off, a falloff map is basically a way to vary a color based on a series of functions. One such function is called Perpendicular / Parallel, which blends between two colors based on the surface normal direction, so edges of an object can be a different color then the front. Another function is Shadow / Light that lets you change color between the dark and lit part of your material. Using these maps, and layering them together using mix, you can make a color layer that's based on Shadow / Light, a highlight layer based on the same thing, and an inked edge based on Perpendicular / Parallel.
Here's the max file that made the image above, max4: toon_falloff.zip
The key is to adjust the output graph on your Falloff, making the transition between colors very sharp, to get that hard edge transition between bands of color.
A similar output trick is used to get a sharp edge, and a sharp highlight. For complete details on how the material works, please check the file. Also, since most renderers has some sort of variation of the falloff map, this technique can be adapted to just about any rendering system available.
Please note that a real toon shader can do a lot of things that this method cannot, especially when it comes to inking. A toon renderer has true edge detection, as opposed to just a color change based on normal direction, which gives it a lot more options. For example, it can tell if an object intersects itself, changing the color of the edge based on self intersection. A true toon renderer will also give you better and more consistent results, especially when animating. However, I have found materials such as this one useful in the past, and figured I'd pass it along.