Imagine the scenario: you've got dozens of photos scattered across your hard drive, all in a variety of sizes and file formats. You want to place them all in the same folder, in the same format and perhaps even rename them for good measure. Perhaps they all need to be the same size, or you'd like to add a watermark. What are you going to do? You could open each file individually and perform the same repetitive actions on each, or you could go looking for a tool to do the hard work for you.
FastStone Image Viewer is - as the name suggests - a tool for viewing images, but that does it a massive disservice. It features some nifty editing tools, plus has a contact sheet builder, enabling you to quickly and easily print out thumbnail previews of a collection of images. But where the program really stands apart is with its batch-processing capabilities. It enables you to quickly convert a group of selected images from one format to another. You can save them all to a different folder, and rename them too.
Tick the Advanced options box, and you open up a whole wealth of options: resize, rotate, crop, change colour depth and DPI, adjust lighting and colours, add a border, and watermark your images with text or an image. Once you've set things up, click Convert and watch the program do in seconds what would take you minutes or even hours to achieve on your own.
Note that this is the portable version of FastStone Image Viewer.
You can take or leave the program's viewing and editing options, but as a batch conversion tool - that's free for personal use to boot - we've yet to find an equal.