photoshop -> filters -> find edges -> some tweaking of the tone curve
I was being deliberately vague because I don't know exactly how the person who made the example processed it, but it's probably this. Find Edges and then maybe something else to clean it up or make the text stand out.
Someone else linked the image and its original file name was "gimp-2.png" so it wasn't Photoshop :P
Doing it in Gimp is easy. Load the image; Filters->Edge Detect->Sobel