An image viewer for Linux and other *NIX derivatives, characterized by its lack of user interface, save from a fancy right click menu.
It is aimed at console users, thus most of its features are hidden behind command line switches and keystrokes. It is also capable of light image editing, like rotating, flipping, or blurring, as well as making what it calls a "montage". Don't get too excited, it's just thumbnails in a grid overlaid on another image.
While not appropriate to manage a large collection of pictures, it is very lightweight and quite nice for quickly checking out an image.
It is pre-packaged in most Linux and BSD distributions' package managers, and its source can be found at http://feh.finalrewind.org/.