There is a technique that I have seen in some specialty software that can vectorize an image using a single color such that is has a photo-realistic effect. This can be useful in some applications like engraving, etching, and cut vinyl.
Here is an example of what I am talking about.
Until recently I was just aware of commercial software able to vectorize in this manor.
But today I have found ZebraTrace and have been experimenting. It is open source. It outputs SVG files that are vectors and not embedded bitmaps. It can be found at http://maxim-s-barabash.github.io/ZebraTrace/. A guy that is in the #inkscape chat room on freenode pointed it out to me.
Just thought I'd let you guys know about it. It's a nice tool to have handy I think.