V 3.0: Lens flares & dirt, visor reflections / water droplets, god rays, depth of field, night & thermal vision scopes, dynamic wet surfaces with reflections, rainbows, volumetric light & fog, colored speculars and many other effects.
Easily an essential part of the immersion experience.
Goes well above the stock rendering settings and compliments/interacts with replacing the Complete 1.4.4 shaders entirely and has more options than you can throw a bolt at, but they are well documented in their respective settings files. With enough time and some light reading, anybody can configure their experience exactly as they want it to be.
Hands down, I can't play STALKER without this, and neither should you.