So your looking for a free piece of software to convert a raster image to a vector image? I didn't even know that these was technically possible until I searched for this myself, and frankly I was amazed. Maybe that was my ignorance, but I am well aware of the differences between a Raster and Vector image and I still cannot comprehend just how this works.
That said, I had half an hour messing about and the results were fantastic - so what I can tell you is that it most definitely does work! This is a godsend for web developers, for obvious reasons. For those of you who have stumbled across this article in some sort of strange unconventional manner, I will first explain the basic differences between a raster image and a vector image.