potrace - Open source program that transforms bitmaps into vector graphics.
ImBatch - ImBatch is a batch image processor with a nice graphical user interface.
Vectorizer.io - Fast and easy way to vectorize an image online
DVDVideoSoft Image Convert and Resize - Free Image Convert and Resize is a compact yet powerful program for batch mode image processing.
Autotracer.org - Free online image vectorizer
Ralpha Image Resizer - High-speed image batch conversion tool