Boris have released a free, stand alone, version of their particle generator, Particle illusion. Particle illusion is a great way to generate lots of particle effects. It is part of the Boris Continuum suite of effects which plug into various programs like After Effects, Avid etc.
As a plug-in you can get it to interact with other elements in your video, more easily than you can with the stand-alone version but there is a lot you can do with the free standalone. When you have made your particles just export and movie and composite them in your editing program such as EDIUS etc.
You can get the free version and see some tutorials on how to use PI here: