For this part of my blender particle experiments I used dynamic paint to generate an effect similar to the reactor particles from blender 2.49
I used 2 particle systems, the first one is setup as a brush for dynamic-paint, and influences a vertex-group on a plane.
The second one is a hair-particle system that uses the vertex-group for length and density of the created particles.
so I can create a bunch of smaller particles where the big ones hit the plane.
you can download the blend file here
blender particle experiments - dissolving tree
blender particle experiments - keyed particles
blender particle experiments - wobble
Blender particle experiments - particle-info node