Windows Explorer's preview pane and image thumbnails are nice, but if you need a larger preview of an image you usually need to double-click on it and open it up in a program. The problem is, if you ever work with images, you've probably already changed the double-click behaviour to open in something useful, like the GIMP, instead of Microsoft's Image Viewer.
Instead of going through the "Open With" menu every time you open an image, FastPreview adds an image preview to the context menu. From there, you can see the size of the image and a larger thumbnail, and clicking on it will open up a super fast, full-size preview of the image, almost like OS X's oft-envied Quick Look function. It also adds an option to the context menu that offers a ton of information about the photo, including EXIF data and other tags. You can also change the resolution of the context menu's thumbnail, making it even easier to see the image. If you work with images at all, it's a great little addition to the context menu.