Create cracks in your images randomly using default Gimp filters.
Combine a white to black gradient with a wind filter to form a brushed metal look.
Give your animation characters the effect of having bones.
Put text along a path or circle, warped/fuzzy characters, drop shadows.
Interpolation is a useful way to change from one shape to another across distance.
Give your text a classic, outer space, science fiction look.
Following this tutorial will show you how to put fractals to use by creating branches for a tree.
Create a "connect the dots" or "rhinestone" pattern using Scatter effect (rather than Pattern Along Path).
Make a square shape with patterned edges.
Whiten teeth in photos using desaturation.