Qiewt is probably the tiniest, simplest image viewer out there. It displays any image thrown at it (i.e. dragged and dropped on it) on your desktop - just the picture, no borders, no frame, no toolbar.
Move it around with your mouse or keyboard. Scale it to different sizes. Read on for a complete feature list.
Now... what's it for?
Why, you may ask, do I need something that simple? Life is more complex than that!
Well, I don't know what uses you may come up with, but I do web design and I often need to have a quick look at whether some graphic fits into some existing web page or whether some image slices for page backgrounds fit well together. Without Qiewt I would have to take a screenshot of the webpage, open it in a graphics program, add the other graphic, etc.
Qiewt is written in Python using PyQt4. So everything you need is a recent version of Python, PyQt4 and Qt4 which are already included in every up-to-date Linux distribution.
To run Qiewt, just make sure the mentioned prerequisites are installed, download qiewt.py, make it executable, click on it (or something equivalent) and there you are!