DX9 Particle Systems
Particles (2d Field)
Particles (2d ForceField)
Particles (3d Field)
Besides the nodes from the Addonpack contributed by vux (see left column) there are two powerful particle system contributions:
There is also this very comprehensive guide written by dottore about DX9 GPU particles.
This forum thread is an interesting read about CPU-based particles. And vvvv's c#-utility library comes with a simple Particle Class which should be a good start for simple CPU particles.
And don't forget the related contributions by other users: