Autotracer.org - Free online image vectorizer
Sketch - Professional digital design for Mac.
Vectorizer.io - Fast and easy way to vectorize an image online
Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
potrace - Open source program that transforms bitmaps into vector graphics.
Adobe Photoshop - Adobe Photoshop is a webtop application for editing images and photos online.