Create light rays in Cycles I am trying to create something like volumetrics with a falloff: beams of light that shine visibly into the air from an emissive object, becoming gradually less visible as they get farther from the object.



This strikes me as involving some sort of volumetrics, which Cycles supports (yay!), but extruding the actual geometry out and giving it an emission volume shader either produces extreme blocks of light or none at all (and it doesn't have that cool flame appearance). 

Plain text on a plane would be perfect!
Using a Gradient texture you can fade the volumetric emission off however you want:



In this case, since it should fade out spherically from the center of  the ring I used a spherical gradient:



The vector input of the gradient node defines the position and size of the resulting gradient.
Since in this case we want the center of the gradient to be in the center of the ring, the Object Coordinates (0,0,0 at the object's origin) are what we want. The multiply node scales the coordinate system, which lets us control the size of the gradient.

In the image above the rays are made up of an extruded text object:



For linear gradients, you might not even need the gradient node. For example, the Generated coordinate system has 0,0,0 at one corner of the object's bounding box and 1,1,1 at the opposite corner. Using this, you could fade an object out along it's length by using one of the axis as the gradient: