potrace - Open source program that transforms bitmaps into vector graphics.
Vectorizer.io - Fast and easy way to vectorize an image online
DragPotrace - A GUI for potrace using Objective-C for OS X 10.11
Ras2Vec - Raster to vector conversion program for Windows 95/98/NT
Autotracer.org - Free online image vectorizer
RasterVect - Professional raster to vector conversion program (vectorizer).