Around this time last year, when my wife and I were looking to give some money to charity, I was struck by a sudden utilitarian panic. I wasn't giving away very much, so how could I know that my small contribution would amount to anything? What if the money just went to some charity's overhead or marketing? Or, perhaps even worse, what if I gave money to a charity that was efficient with its resources but was dedicated to a cause that wasn't the most worthwhile use of my money?

So I asked for help on Twitter. Within a few minutes, someone sent me a link to GiveWell, a nonprofit charity-review organization that has completely changed the way I think about donating money.

GiveWell studies charities the way a stock analyst might research companies. The group was founded in 2007 by Holden Karnofsky and Elie Hassenfeld, who were analysts at the hedge-fund Bridgewater Associates and had been stymied by a lack of online guidance about how to give. "We wanted to do the same thing for charitable giving as we could do for any online purchase—get the best deal for the money we were giving," Mr. Hassenfeld says.

After raising $300,000 from their colleagues, the pair founded GiveWell and began a gargantuan research task, studying academic articles and reams of data from hundreds of charities around the world, as well as conducting many interviews. Every year since then, GiveWell—which now has 11 full-time researchers—has produced a short list of "top charities," groups it believes will use your donation to produce the largest impact in the world. The recommendations reflect not just the staff's intense investigations but also its attempt to grapple with a series of vexing, nearly unanswerable questions about who in the world might benefit most from more money and how.

GiveWell sits at the center of a small but growing movement in philanthropy, what you might call "evidence-based giving," which is particularly in vogue among tech millionaires and billionaires. In addition to praise from economists and those in the traditional philanthropic world, GiveWell has earned accolades from tech types. Its largest funder is Good Ventures, the foundation created by Facebook co-founder Dustin Moskovitz and his wife, Cari Tuna, a former Wall Street Journal reporter who now runs the foundation. In its philosophical outlook, GiveWell also has much in common with other tech-funded philanthropies, including Pierre Omidyar's, Jeff Skoll's and especially Bill Gates's. Indeed, GiveWell recently moved its headquarters from New York to San Francisco, in part because its philosophy has found more traction with donors in Silicon Valley than finance types on the East Coast.