FastStone Image Viewer
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Full user review
"Excellent, full-featured viewer and much more."
Used primarily as a viewer this program cleverly allows full-screen (and mouse-click zooms) views, scaled by (custom) percentage to suit your own megapixel (ie: size of image). Low-level image adjustments available from within, exif data, etc etc
Very strong on image quality - seems to 'bring out' what you've achieved with your camera (perhaps more than IrfanView does).
To get this quality for free is rare indeed. Do not hesitate, but give it a couple of weeks to get used to the options / layout