This very cool trick comes from Saul Goode, courtesy of the GIMP User Mailing list. To create a zig zag pattern very quickly…
Select the top half of your layer, enter Quick Mask mode, and run Filters->Distorts->Ripple with the Smear, Vertical, and Sawtooth options (use the sliders to size your zigs and zags). Exit Quick Mask mode.
Here’s an example result:
