This plugin evens the tone of an image. It useful in photography for correcting weird light, and can hide skin blemishes.
The layers are not merged to allow opacity adjustments after the filter has been run.
Usually for a photograph, you'll want to use either the sharpen layer, or the "Soft Light" layer. Using both can make colors look too saturated, and has a posterizing effect.
For extra sharpness, change the sharpen layer mode to grain merge, but be ready to lessen the opacity, as the effect is quite striking.
For fixing skin blemishes, turn the sharpen layer invisible.
As suggested in the comments, the menu entry has been moved to Filters->Light and Shadow. (Thanks for the suggestion!)
A saturation layer has been added to tone down strong colors on some images. Usually the opacity of this layer will need to be dropped to around 50% or so.