Self promotion: "Edge Finder" applies an edge detection algorithm on the camera preview. Falls into the useless (but neat in my opinion) category.
Also source is available :)
It is free. I just thought it would be a fun experiment while learning the android SDK. It ended up being more difficult than I thought it would due to how the camera preview works. There doesn't seem to be any way of getting the camera preview without displaying it on a SurfaceView. The only way I could figure out how to get it to work was to put a View on top of it to draw the edge image on. I also tried making the View I draw on top a SurfaceView which allows for more control of when you update the view but you can only have one and for some reason that I don't understand I only ever saw the camera preview when trying this.
The algorithm itself is just the most basic edge detection algorithm I could find: http://en.wikipedia.org/wiki/Edge_detection#Other_first-orde...
I'm not entirely sure if that is what you were asking. It just requires the android NDK and SDK to build it. The source is at http://github.com/bwr/edgefinder