I think lighting is the most important thing in CG, you can have the best models with the best textures, if the lighting is wrong, then all is wrong. But an average models with so so textures and a good lighting can look so much better.
Lighttune lets you specify lighting per object,  you can add speculars to the eyes of a character to bring life to them without any diffuse light, you can specify light fallof filters to match real world lights, and you have the shadow map option which can save you a lot of rendering time.
And if I remember right, it's the cheapest module!
Go get it, you know you want it!