"What it does: Sells renewable energy — wind, solar, hydro, geothermal, and natural gas — to 600,000 consumers nationwide. Why it's growing: Thanks to deregulation, consumers have far greater choice over where they buy their electricity. And thanks to recent technological advances, especially in wind power, the price of green energy is competitive with the price of energy generated by coal-fired plants. "We give people a choice of where they get their power from," says CEO Paul Thomas. Breath of fresh air: By using renewable energy, Green Mountain claims, customers keep 623,000 tons of carbon dioxide out of the air each year."