1. First Solar
The "Google of solar" is the first renewable-energy company to be added to the S&P 500. Top 50 No. 6 
California's largest utility was the first company to quit the U.S. Chamber of Commerce over its opposition to climate-change legislation. Top 50 No. 7 
Formerly Sustainable Spaces, it's one of the nation's leaders in home-energy audits, retrofits, and systems. Matt Golden, the tech-minded, laser-focused founder and CEO, has been the key architect of Obama's Cash for Caulkers, a proposed $23 billion two-year program to green America's homes.
4. NextEra Energy Resources
NextEra added another 1,000 megawatts of wind power in 2009 and has more than 600 MW of solar-thermal projects in California and Arizona in the works. It also raised $22 million in a trust, from consumers and corporate partners such as Citigroup, Office Depot, REI, and HSBC, for its clean-energy projects, issuing renewable-energy credits in return. Innovation All-stars 
In mid-September, the company unveiled a plastic-to-oil technology 15 years in the making. Using a proprietary infrared process, one of its 47-by-13-feet units can convert 10,000 tons of plastic trash per year into up to 50,000 barrels of synthetic oil for less than $10 per barrel--for a net energy gain.
The world's wind leader was not immune to the downturn, with a slowdown in the rate of profitability growth, but it's still expanding and gaining on a "Triple 15" target: 15% operating earnings on revenue of 15 billion euros by 2015. Vestas is also making its award-winning internal education program on wind and climate change available to the public. Innovation All-stars 
7. Recurrent Energy
Rather than fight wilderness advocates to put utility-scale solar in the desert, Recurrent invests in fleets of small- to mid-scale solar-power installations  close to cities and other high-demand areas. In September, it announced a 4.8-MW solar-power project in Spain, to occupy rented rooftops, with a real-estate investment trust. Construction has begun on a project to solar-tile the roof of San Francisco's largest reservoir--the size of 12 football fields--to generate 5 MW of solar power. It's expected to be completed early this year.
Its systems embed directly into solar panels , allowing real-time monitoring and improved conversion using smart-grid-like technologies. The result can boost solar-power systems' output by up to 25%. In October, the Israeli startup closed $23 million in an international round of funding that included venture capital from GE.
9. Silver Spring Networks
The most widely adopted smart-grid platform in North America works with utilities that together serve 20% of U.S. customers. Its projects encompass demand response as well as smart-meter, smart-home, and smart-garage projects. In April, Silver Spring had a notable launch of its Energy Smart Miami program, a collaboration with Florida Power & Light, GE, and Cisco Systems.
The Pune, India-based company is the wind market leader in Asia. Quality-control issues cost it sales in 2008 and the recession reduced demand in 2009, but it's still the third-largest turbine manufacturer in the world, with a 12.3% market share. Suzlon distinguishes itself  with a vertically integrated, turnkey structure. Suzlon will site large projects, get the permits, acquire the rights, and construct the power lines, the buildings, and even the roads leading to remote areas.