Particle Tracer is a free filter for Photoshop, and anything else - like Paint.NET - that supports 8bf files (it won't run stand-alone),
The developer's technical description is that it "traces particle trajectories", but effectively this just means it creates gorgeous images which look huge electrical storms. In deep space. Or something like that. (Take a look at the grab for an idea.)
You're able to customise the final image by tweaking a lot of variables, including the number of particles, their lifespan, colour, density, brightness and more.
Fortunately you don't have to worry about any of the technical details, as everything is controlled by an assortment of buttons, sliders and listboxes. Click, drag and choose various options at random and you're sure to find something you like.
An easy-to-use particle generator, capable of producing some spectacular effects.