GiveWell does extraordinarily rigorous research on which charities and causes will do the best with your money—the best we've seen. They make sure that their recommended charities do what they say they do, that what they say they do is effective, and that they have room to use your donation productively.

Giving What We Can is a society of people who have pledged to donate 10% or more of their income to the most effective causes.

80,000 Hours researches how we can choose careers that will have the most impact. They also provide one-on-one career coaching for aspiring altruists.

The Life You Can Save, an offshoot of Peter Singer's book by the same name, is the largest effective altruist organization. Its 16,000 members have each pledged to donate at least 1% of their income to effective charities.

Effective Animal Activism investigates the most effective ways of reducing non-human animal suffering. Since there are many non-human animals living in extremely painful conditions on factory farms, animal suffering could be a very highly effective altruistic cause.

The Centre for Effective Altruism is an umbrella orginzation running 80,000 Hours and Giving What We Can; Effective Animal Activism and The Life You Can Save are also affiliated.

The Future of Humanity Institute uses the tools of mathematics, philosophy, and science to research big-picture questions about the future of humankind, including the altruistic potential of working towards improving the lives of people in the far future.

Additionally, many individual writers and other sites have interesting material on effective altruism:

Our sister organization: Yale Effective Altruism