Fragment is a fast and lightweight image viewer.
Launch the program and it displays precisely nothing at all. That's normal, though; if you drag and drop folders onto the Fragment window, or click "no image" and browse to the folder with your pictures, then the first image will be displayed (all the basic formats, plus animated GIFs, PSD and HDR - but not RAW - are supported).
The program has no immediately obvious interface, no toolbars, menus, buttons or anything else. But if you move your mouse cursor to the top of the Fragment window, then various navigation options appear. "Back" and "Next" buttons. A "Browse Panel", with resizable thumbnails. And an "image indicator", which provides a quick and easy way to browse through a folder (it's something like a scroll bar, and can zip through your thumbnails just as fast as you like).
Right-clicking displays options to open your image in your choice of application, upload it to Facebook, or show its folder in Explorer.
Fragment also offers good support for common mouse and keyboard actions. Spinning the mouse wheel zooms in and out; clicking and dragging moves the image around; the right cursor key (or space bar) moves to the next image, the left moves back, Alt+Up goes up a folder, and Esc closes the program.
The lack of any menus or toolbar buttons means it takes a little time to figure out how things work. When viewing an image, for example, a narrow bar appears to its left. You'll probably ignore this for a while, but if you move your mouse over it, you'll see your current zoom level and an icon. There's no clear indication what these might do, either, but clicking the zoom figure switches to a 100% view, while the icon controls the default zoom view ("Fill" or "Fit").
This can be a little frustrating, especially when you're left wondering how to do something as simple as open a new folder. The point-and-click route isn't immediately obvious (move your mouse to the top centre of the screen, hover it over the image name, and click the current folder name). And although in theory you can use the usual Ctrl+O Windows shortcut, in practice it didn't work every time (we've no idea why).
Still, once you've learned the Fragment basics, this isn't a significant issue. And if you like the program, its lack of any visible interface becomes a real plus; there's no clutter to get in your way, and you have the maximum possible space to view your images.