ImBatch - ImBatch is a batch image processor with a nice graphical user interface.
Google Open Source - All of Googles open source projects under a single umbrella
DVDVideoSoft Image Convert and Resize - Free Image Convert and Resize is a compact yet powerful program for batch mode image processing.
LaunchKit - Open Source - A popular suite of developer tools, now 100% open source.
Batch Image Resizer - Resize, crop, shrink, flip, exif-rotate, convert, enhance, process multiple pictures and photos with professional software! 120+ Actions, 30+ Image Formats
Open Source @IFTTT - A collection of IFTTT OSS projects.