Version 2.0 of my old Paper Texture Script, now more parchment like and with weathering and black spots.
Install in your /.gimp2.6/scripts/ directory or the Windows equivalent directory in Users/derpasnerp/Application/GIMP/scripts/ or whatever it was, and then reload the scripts in GIMP to find this one in Decor > Parchment Texture.
This script has been optimised for medium-sized images; it looks best around 1000x1000, but if you go bigger you need to start do some manual grunging.
How to use: make your image, then run it through this script. A duplicate image of your original will be created, and it will be run through the parchmentification process.
Not really useful for photos, mostly this is meant for images on white backgrounds consisting of large areas of uniform color or text.
The script uses the native Galaxy, Big brush, so make sure you have that, and it also uses the Lava script-fu script, plus some other plugins.
The preview above is Before/After.
The result (if you haven't toggle flattening) is an image with all the layers and layer masks intact, so you can easily edit the result if it's not exactly right. I mostly suggest playing with layer opacity and the plasma'd layer masks with blurring and leveling.