Comparometer
This tool lets you compare reference images from digital cameras side-by-side. These images are JPEGs straight from the camera, and were taken under carefully-controlled conditions, to provide valid comparisons of camera capabilities in actual shooting situations. You can also download the images (using your browser's "save image as" function) and output them on your own printer, to see how the cameras involved will perform in your application. (See the copyright notice first though!)