(October 7, 2017)
Darkulator is a program for adjusting the light levels in a GoldSrc map. After loading up a BSP, one can scale the lightmaps to be either brighter or darker. There are no fine tuning controls, only a slider. The program also lets you change the skybox texture in a map.
The Darkulator documentation included with this mirror recommends saving a few different versions of any map you modify with the tool and comparing them in-game to make sure the changes are satisfactory. Darkulator does not work with Half-Life: Blue Shift maps.