It's basically a Gradient Ramp map combined with the Falloff map and particle-based effects (the standard age/speed/size) with great tweakability. However, the best part - that makes it on-topic for this forum section - is that it is scripter-friendly.
With the Gradient Ramp map, you could never add keys, remove keys, set key info, etc. With Bercon Gradient, you can.
I posted a little snippet of code with sample images up on our Forums, have a peek there:
The plugins are absolutely free (as in beer), but if you like the map(s), why not make use of the paypal button on his site? :)