Nonprofit organizations, or NGOs, have become a growing tool for companies as they strive to reduce their environmental footprints while garnering business value.

NGOs have invested heavily in building expertise in a range of areas, while at the same, the animosity that once existed between NGOs and the business community has steadily thawed. Many now view NGO-business partnerships as the rule, rather than the exception.

The type of NGO chosen by a company often depends on the sector, said Thomas Lyon, editor of the new book, "Good Cop/Bad Cop: Environmental NGOs and their Strategies Toward Business." "There's no one-size-fits-all for which NGOs to work with," Lyon told us.

Who are the NGOs that companies turn to most often -- and most effectively? To answer that question, we spoke with companies with long experience working with nonprofits, scanned our coverage over the last 10 years, and compared notes. Several NGOs stood out for their efforts to show the business community how they could operate leaner, greener, more profitably and more transparently.

While we'd love to offer a ranked list of the absolute best NGOs to work with, there are too many groups out there working in too many areas. And although some of the NGOs listed below received a number of recommendations, they're listed in alphabetical order for simplicity. We'd love for you to share your suggestions for others that have helped you make a difference in your operations.

1. Carbon Trust -- The U.K. government established the Carbon Trust as a nonprofit to help the business community move toward a low carbon economy. Its menu of resources is broad and seems to keep growing with programs aimed at companies both large and small.

For example, it helps big companies create climate change strategies, gives small businesses free energy audits and no-interest loans for energy efficient equipment, and even funds budding cleantech entrepreneurs. Its work with product carbon footprinting contributed to PAS 2050, a product life cycle standard tested by the likes of PepsiCo, Cadbury and Coca-Cola. Though based in the U.K., look for a growing Carbon Trust presence in the U.S.

2. Ceres -- Since 1989, Ceres has brought together investors environmental and public interest groups to promote corporate responsibility with the goal of getting the marketplace to recognize and reward companies that incorporate sustainability into their business practices. It works year-round with its network of companies that have committed to sustainability reporting, stakeholder engagement and continuous improvement. It also takes on environmental issues at the sector-level, such as the oil, electric power, insurance and hotel industries.

Ceres holds the bragging rights to several important achievements: it developed the Global Reporting Initiative to help companies report sustainability performance in the same way as financial information; it launched Business for Innovative Climate and Energy Policy with Nike, Gap and others to give companies a larger voice to push for aggressive climate change laws; and it produced the "21st Century Corporation: The Ceres Roadmap for Sustainability," a set of 20 business goals it says will be expected of all companies by 2020.

3. Clinton Climate Initiative -- The William J. Clinton Foundation created the CCI to battle climate change by making energy-hungry cities more efficient, ramping up green power generation, and keeping the world's carbon-storing forests standing.

You'll find opportunities to work with the CCI largely in the areas of buildings and waste. Its work partnering with governments, NGOs and businesses includes a building retrofit program that can helps developers and owners improve the efficiency of their buildings, such as the high-profile retrofit of the Empire State Building. Several commercial structures in Bangkok, Mumbai, Chicago and New York are also getting energy face-lifts. The CCI also joined forces with Walmart to curtail the amount of waste it sends to landfills in Houston through a compost program that now includes participants from more than 40 local grocery stores.

4. Conservation International -- It has crossed paths with dozens of companies through its efforts to preserve and protect biodiversity since its founding in 1987. Conservation International (CI) offers two main corporate partnerships: It is well-known for inviting companies to donate to projects not directly related to their business, such as forest protection. It also helps companies create best practices to reduce their environmental impacts, such as helping Alcoa add biodiversity conservation into its environmental policies, advising Starbucks on a range of sustainable issues related to coffee, and working with Walmart to track sourcing for its jewelry products. With new tools on the horizon, CI in also interested in playing a larger role in influencing whole industries toward better green practices, rather than one-off efforts with individual companies.

5. EarthShare -- Its member charities founded EarthShare in 1988 as a facilitator of workforce giving programs to support environmental charities not represented by traditional programs -- sort of a United Way for green groups. The organization now has on-the-ground representation in many states, with 14 state affiliates offering dozens of local giving options and a network that helps companies engage their employees and boost morale.

Over the years, the organization has evolved in lockstep with a changing marketplace that has demanded guidance on growing number of green activities, such as launching green teams, communicating goals internally, and cause marketing opportunities. Now EarthShare is creating package of services to help workplace partners bring their employees on board for green initiatives, support workplace sustainability goals and provide volunteer opportunities.