I knocked this up rather quickly, but it does its job. It's a poor man's edge detection node. You can choose the intensity of the effect and one of three different neighbourhoods (Moore, Von Neumann and a 2D neighbourhood). There's also two variants of Moore (normal and "skinny", which removes one direction to provide a half-way house between that and Von Neumann) and two variants of the 2D neighbourhood (vertical and horizontal). The sensitivity is divided by three to give finer tuning, as quite often only the lowest values are particularly useful.
Here's the link: Differ.
And here's an example:
