Boids particle systems can be set to follow basic rules and behaviors, something close to artificial intelligence. They're useful for simulating flocks, swarms, herds and schools of various kind of animals, insects and fishes.
Blender's Boid Particles in Action:
For many more details, go here. Be sure to check out a step-by-step tutorial, created by olivS, on How to setup a prey-predator relationship using Boids particles.
The new particle system can be tested by installing one of the experimental builds available at graphicall.org.
Note: Do not use experimental builds for mission-critical projects or files. Although tested, they are still unofficial releases and are subject to change at any moment.