Any Freeware Alternative to Lightroom?

Would anyone know of a freeware post processing application that has the "Fill Light" function of Adobe Lightroom?

This function allows you to add in a correction to a photo that simulates a flash being fired for photos that are too dark.

Changing the exposure is not acceptable because it alters the overall brightness and contrast of the picture. But the "Fill Light" of Lightroom merely modifies the image to appear as if a flash fired when the photo was taken.

Thanks for your help.