I use GIMP for a lot of things, but mainly making thumbnails for my YouTube videos, and there are a lot of GIMP tricks that I'll probably forget, so might as well just put 'em here.
Plasma
For a quick background you can generate a plasma. Select (or create) the layer you want to be filled with plasma, then click Filters > Render > Noise > Plasma, you can regenerate the random seed until you get a good plasma, then also you can scale up the layer if you want a specific portion of the plasma.
