digiKam - Professional Photo Management with the Power of Open Source
ImBatch - ImBatch is a batch image processor with a nice graphical user interface.
XnView MP - XnView is a free software that allows you to view, resize and edit your images. It supports more than 500 different formats!
DVDVideoSoft Image Convert and Resize - Free Image Convert and Resize is a compact yet powerful program for batch mode image processing.
FastStone Image Viewer - FastStone Image Viewer is a fast, stable, user-friendly image browser, converter and editor.
Ralpha Image Resizer - High-speed image batch conversion tool