In 1994, GuideStar began as a new venture with a bold vision: to revolutionize philanthropy and nonprofit practice with information. (www.guidestar.org)

In their journey to create a more transparent and accountable nonprofit community, they are reshaping the idea of the non-profit from top to bottom. Nonprofits embrace this practice of transparency and accountability when they share information about their mission, programs, and finances with the public. Accurate information is a powerful tool, and their users have responded with better-informed, more effective giving decisions.

When they first began, they envisioned GuideStar as a resource that would connect people and organizations with valuable information to improve the world around them. Today, GuideStar is the leader in providing comprehensive data on more than 1.5 million nonprofit organizations, connecting them with donors, foundations, businesses, and governing agencies in a nationwide community of giving.

Each day, about 20,000 people find answers using GuideStar. The information we provide contains searchable data from IRS Forms 990 and the IRS Business Master File, including comprehensive facts on employee compensation and grant activity. With our new eDocs service, nonprofits can add their annual reports, audited financial statements, letters of determination, and other documents to our growing store of information. This is making GuideStar a more powerful tool than ever.

There are a number of new organisations that aim to increase the transparency and knowledge about charities. Charity Navigator is an example of another US organisation that rates charities. Development Ratings is a UK based service that uses GuideStar UK and other sources to analyse charities and pick ones to recommend to donors.

Ref: http://www.guidestar.org/help/faq.jsp#operations
http://en.wikipedia.org/wiki/Guidestar