I am particularly interested in interactive particle systems and have been playing around with them using Quartz Composer.  This sketch is intended to simulate fire and uses a gravitate force to attract particles to the mouse location.

Quartz Particle Fire from Noah King on Vimeo.

This is a Quartz Composer composition, exported to video using QuartzCrystal by Kineme.

A particle system, which is rendered to resemble fire, follows the mouse around using attraction/gravitate forces.