As I really quite like Photoshop's Adjustment Layer feature, I thought I'd give porting it to The GIMP a try. At the moment I succeeded in creating these nine Adjustment Layers:

Brightness
Color Overlay
Desaturate
Dodge or Burn
Gradient Fill
Invert
Lightness (positive only)
Pattern Fill
Solid Color

Previously I had submitted a subset of these under the name "Semi-Adjustment Layers". I changed the name, as I improved the functionality. You can now easily re-adjust an adjustment layer by running the same script on a layer twice. If you do this though, make sure to select the "Replace Layer" option. This will replace the current layer with the new adjustment layer, but will maintain the layer mask, the previous layer's opacity level and the layer's link state. I also moved the scripts to the Adjustment Layer submenu, making the scripts easier to access.
As with the old version of these scripts, when you apply an adjustment layer the current selection will be transformed into the adjustment layer's mask, that way making sure the effect only applies to the selection.

Hope you will find this useful