EmbalonView
EmbalonView is more powerful than the viewer which is part of Windows.
EmbalonView is a small an fast image viewer, who aims to be similiar to the older versions of ACDSee.
When are you part of the target audience of EmbalonView:
When you want a fast image viewer, who does not need 10 seconds after a double click in the explorer before you see your image, because it took so long to load itself. When you want do move/copy your images into directories with only pressing a few keys (Return, chose one of the previous targets with arrows, or enter/chose new, Return, Done). The sidebar gives you a overview about all current images (usually the images of a specific directory or directory structure on your hard drive, or from a website).
When you might not find EmbalonView sufficient:
When you often want to do more with your images than watching and sorting. EmbalonView is nor a image processing tool, neither it will become one. Altough there is the mentioned option to rotate or mirror images.