Vectorizer.io - Fast and easy way to vectorize an image online
potrace - Open source program that transforms bitmaps into vector graphics.
Autotracer.org - Free online image vectorizer
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!
Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
vectorization.eu - Free online vectorization tool that can help you turn black&white and colored images into a vector format.