PicaView!

SageThumbs is a powerful shell extension allowing to preview enormous amount of image formats directly in Windows Explorer by using Pierre-e Gougelet's GFL library (XnView's author).

XnShell is a free File Explorer extension that installs a number of image-related tools in the right-click context menu, including a thumbnail preview, image conversion to a wide variety of formats, image resizing, editing image IPTC metadata, uploading to ImageShack, and other functions.

Pictus is a fast, free image viewer that enables Windows to see thumbnails of supported formats, such as PSD. Give it a spin today!

Image Resizer for Windows is a utility that lets you resize one or more selected image files directly from Windows Explorer by right-clicking.

FastPreview was created in need of a usable PicaView replacement which was discontinued at the time.

Photo Size Optimizer for Mac:

FirmTools ShellExtension is a real handy freebie for Windows users.

Context Viewer is an explorer extension in the form of a Control Panel applet that lets you preview image files and text in the right click menu. It can also display customized information about images and set hotkeys to enable or disable the preview along with a few more options.

AhaView is a handy picture viewer and converter. It allows you to browse, view, organize and convert your digital images.

This shell extensions will show you a thumbnail of an image file, as well as its dimensions, when you right click on a filename inside Explorer.