potrace - Open source program that transforms bitmaps into vector graphics.
DVDVideoSoft Image Convert and Resize - Free Image Convert and Resize is a compact yet powerful program for batch mode image processing.
Vectorizer.io - Fast and easy way to vectorize an image online
Batch Image Resizer - Resize, crop, shrink, flip, exif-rotate, convert, enhance, process multiple pictures and photos with professional software! 120+ Actions, 30+ Image Formats
Autotracer.org - Free online image vectorizer
Ralpha Image Resizer - High-speed image batch conversion tool