This application is more than its name implies. Besides allowing you to preview your photos, this program also enables you to convert, edit, rename or compare your favorite pictures with minimum effort.
FastStone Image Viewer works with a wide variety of image formats, supports batch processing, and gives you access to multiple editing options. Even though, at first, the app's main window might seem overwhelming due to the amount of options, buttons, and panels, you will easily learn the ropes of the interface. Every built-in editing tool is available in the main menu of the utility and comes with easy-to-configure output settings.
Furthermore, you can browse folders containing pictures by using your keyboard keys. If you right-click an image, you reveal another menu that brings you quick access to various editing options.
The program also comes with a built-in slideshow builder. You can import multiple photos, choose their order, resize them to fit a certain resolution profile, etc. Once you finish configuring the output settings according to your needs, click the "Create" button and wait for the results. Unfortunately, the app supports only one output format (EXE), an aspect which might bother you if you intend on sharing the file with a Mac user (Mac OS X doesn't come with a default app for opening EXE files).
All in all, this image viewer offers you many useful features and is free for personal use. So, you should give it a try if you're looking for an alternative to using your default image manager.