Windows only: First Impression is free and minimal image viewer with a focus on maximising the space used for viewing your images by doing away with command bars and buttons.
First Impression is controlled entirely by keyboard shortcuts and a small menu accessed by right clicking on the image viewing window. Navigation forward and backward through your selected image directory is handled by the space and backspace keys, home and end skip you to the start and end of the directory respectively. You can edit the list of image extensions it will load, what image rendering algorithm it uses, and set the image to scale or display at original size.
A novel feature is that images can be dragged around, similar to previously mentioned Vjpeg. First Impression is freeware, Windows only.