I'm adding this to the post so its not lost in some older threads.
Making a lightning strike.
Make a solid gradient. Experiment with the different types of gradients. A single fade will make 1 lightning strike.
Apply the "Difference Clouds" filter. For a more violent lightning strike hold the alt key while you select the filter. The colors should be set to black and white.
Then invert it. If you choose not to invert it, the "lightning strike" becomes a very realistic crack in cement or some other rocky surface once you apply the level step.
And finally go to the Image menu and adjust the levels.
You can then use the styles applet to add colors, or apply filters
The best thing is that because the Clouds and Difference clouds filter is random you will never make the same lightning strike twice. Even if you follow the same steps and do everything the same.