Assuming you're trying to use it within a GIS-system, QGis does the trick.
It's a free GIS-system, and you can set transparancy layer-based, toggle layers on and off, switch around the order, ...
Whereas - to the best of my knowledge - Arcview or MapInfo's Proviewer do not offer the option to combine layers.
That said, transparancy and opacity are not entirely the same, so I'm not sure which you're after. (opacity lets you 'see through' but still have the original colors in a lighter form, transparancy replaces certain colorshades with 'nothing').
In georeferencing images, I tend to set the 'extra' areas which I don't need to white using a photoeditor, and then set a transparancy filter to set '255 255 255' color combination in QGis to transparent. Works like a charm, but might not be what you're trying to achieve.