If energy efficiency is the low hanging fruit in the energy sector, lighting is the fruit already on the ground.  Energy efficient lighting has seen tremendous market penetration in recent years, largely the result of increased incentives and dropping product costs.  Currently two lighting types have seen the lion's share of retrofits: T5/T8 replacements for T12s and LEDs for most everything else.  T5/T8s are the replacement for tube lighting common to business and warehouse spaces.  The can be retrofitted to fit in the existing fixtures and provide the same light output in a similar color.  They also qualify for utility incentives in many cases as well as tax deductions, making this retrofit very cost effective.  LEDs (light emitting diodes) are also very good solution for the replacement of high powered metal halide lamps as well as other specialized lights.  While still the most expensive light source, they use significantly less energy than traditional lights, last longer, and in many cases produce a better light output and color.  They have also seen drastic price drops recently which has helped make them affordable for the common project.  Since lighting is a significant portion of most businesses and homes, it should always be part of an energy solution.