There is a group of scripts called,
CSS WebSprites: Arrange layers and create a CSS file
http://registry.gimp.org/node/25908

There are 3 scripts. For the first two, Gimp usage is awesome.
Arrange layers horizontally
Arrange layers vertically
Export CSS file

The last one, Export CSS file, unless you know HTML or CSS you may need some direction. I might do a tut.
http://hmagalhaes.eti.br/?p=27

A tute would be great Odin

Export CSS file needs a tut.
Arrange layers horizontally and Arrange layers vertically can be used easily. This is 3 layers, grouped side by side and vertically after running the scripts. Another use would be to quickly add two different images to one, side by side or vertically.