I know I've seen this stamp technique somewhere before, but it may have been in Photoshop....heck, there may even be a GIMP script for it, but I have no clue (EDIT: see note at the bottom for new information about the script). At any rate, I'm sure this technique is not mine, but I was asked how I created the stamp effect for a recent piece of work I did. So, I figured I'd put the steps down in tutorial format, just in case someone else is interested. Sorry if this has been shown elsewhere.
You can view the tutorial here: [link]
brought to my attention that there is a GIMP script for this operation, which can be found here: [link]
This script works in 2.2, but I haven't verified it for version 2.4 and don't know that I will, since I've only used the technique once.