placeholder.pics - Lightest way to include placeholder images in your design
ImBatch - ImBatch is a batch image processor with a nice graphical user interface.
Fill Murray - Custom-sized placeholder images of Bill Murray
Batch Image Resizer - Resize, crop, shrink, flip, exif-rotate, convert, enhance, process multiple pictures and photos with professional software! 120+ Actions, 30+ Image Formats
PlaceKitten - Dead-easy placeholder images. Of kittens.
Ralpha Image Resizer - High-speed image batch conversion tool