An incomplete set of layer styles for The GIMP, including:

Drop Shadow
Inner Shadow
Outer Glow
Inner Glow
Color Overlay
Gradient Overlay
Pattern Overlay
Stroke

To add or update a layer style, simply select the layer style's parent layer - the layer the style will or already belongs to - and call the action. This also works from any of the other layer style layers the parent layer has. The scripts will automaticaly find the correct parent layer and/or layer style layer to update.

To be able to run these scripts, you'll first need to install these two scripts, which function as library routines for the layer style scripts:

Select Layer Fill
Calculate Offsets

The scripts introduce two limitations:
You can now only use 9998 layers in one image
The layer styles get "rasterized" when you duplicate your image