potrace - Open source program that transforms bitmaps into vector graphics.

PhotoPenguin App - PhotoPenguin is a completely free batch photo editor with support for cropping, resizing, adding watermarks, and converting photos to other formats. It works on the user's device, using the capabilities of modern browsers.

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!

ImBatch - ImBatch is a batch image processor with a nice graphical user interface.

Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.

Batch Image Resizer - Resize, crop, shrink, flip, exif-rotate, convert, enhance, process multiple pictures and photos with professional software! 120+ Actions, 30+ Image Formats