Shadow Illuminator
I found the Shadow Illuminator through Metafilter.
From the comments to the Metafilter posting, the Shadow Illuminator apparently implements a simple to use web-based contrast mask: quite possible in Photoshop, and probably not as good looking compared to what a Photoshop guru can do.
Of course, I’m not a Photoshop guru — I don’t know much more than tweaking gamma — so I’m inclined to go along with this website to correct the worst of the digital photography mistakes (or at least the photos with unintended chiaroscuro). The drawback is that the sample photos look a bit “flat” — omnidirectional light sources aren’t really natural. If I register, I can apparently control the amount of filtering actually applied.