SageThumbs
Powerful Windows shell extension allowing to preview images.
...To use it, just open any folder with image files in Windows Explorer, and then right-click a file you want to preview. You'll see the thumbnail immediately in the context menu.
System Requirements:
* Windows XP/2003/Vista/2008/7/2012/8/8.1 32/64-bit
* About 5 Mb of disk space + additional space for cache