potrace - Open source program that transforms bitmaps into vector graphics.
Vectorizer.io - Fast and easy way to vectorize an image online
Vector Magic - Easily convert JPG, PNG, BMP, GIF bitmap images to SVG, EPS, PDF, AI, DXF vector images with real full-color tracing, online or using the desktop app!
Autotracer.org - Free online image vectorizer
RasterVect - Professional raster to vector conversion program (vectorizer).
PNG to SVG Converter - PNG to SVG Converter is a dynamic platform that offers you to convert PNG images to scalable vector graphics without any difficulty.