Check these amazing contributions:

• DX11.Particles contribution by tmp.
• Particle Madness: Workshop and Patches by vux and dottore.

Besides the nodes from the Addonpack contributed by vux (see left column) there are two powerful particle system contributions:

• ParticlesGPU Shader Library by dottore
• Particles Unplugged by Martin Zrcek

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:

• Particles related Contributions