Windows: Nexus Image (no relation to Google or the Nexus line of phones) is a free, sharp-looking image viewer. It loads images quickly, displays EXIF information in a transparent overlay, lets you browse images via keyboard shortcuts, and can even dim your desktop to provide a lightbox effect while you browse.
Nexus Image is a portable application, and doesn't need to be installed in order to run. The app can, however, replace the default Windows Photo Viewer. When you run it for the first time, Nexus Image asks you if you want to associate the app with common image types.
Once it's running, your image previews live in a vertical bar at the right side of your screen, and the image you're viewing is displayed in the centre of the screen, with your desktop background dimmed a bit to bring focus to the image.
You can customise how much your desktop is dimmed in the application's settings, and moving between images and folders is as easy as pressing Page Up or Page Down. You can just as easily zoom in or out on images, or switch back to the desktop to bring up another folder to browse. The app is completely free, and if you've been looking for an image viewer for Windows but haven't settled on one that's quite right for you, it's worth a look.