Secular Tithe
Tithing is the practice of giving 10% of your income and/or wealth to the community. Originally, this meant giving one tenth of your grain, wine, oil, fruit and produce; whatever it was that you gained in the year. This was often collected by the religious leaders as they were central to most communities and also part and parcel of the government. Under Mosaic Law, this tithe was distributed locally "within thy gates" (Deuteronomy 14:28) to support the Levites and assist the poor. In some cultures tithes were voluntary and in others it was manditory. Being so closely related to the religion of the times, its therefore unlikely that atheists or humanists are going to take to this idea readily. However, the practice of tithing to support the poor is on it's face, not a bad idea.
The average church member [from across the denominations] today gives about 2.6 percent of his or her income—a quarter of a tithe—to the church. Evangelicals used to give quite a lot more than mainline denominations. But their giving has declined every year for several decades, and they're now getting very close to the norm. The average evangelical giving is about 4.2 percent—about two-fifths of a tithe. Six percent of the "born-again" people tithe; nine percent of evangelicals do. Applying social pressure to people unable to meet the minimum requirements is standard practice: first get people reliant on the church's social network, then threaten to withdraw that network when people don't hand over enough cash — even in cases where tithing the minimum may deny them luxuries like food and medicine.
Non-theists basically believe that humans are on our own in the world and that we need to make the best of it. This means that non-theists specifically rule out reliance on deities of any kind. This is why it's even more important that we offer material support - our money and our time - to organizations that do good by advancing the values that atheists hold dear. Call it a secular tithe. Secular, non-controversial causes like Doctors Without Borders, UNICEF or Feeding America, as well as groups that specifically advance atheist goals,(freethought and humanist groups) are the best candidates for your giving.
Kiva: A microcredit organization that connects donors with entrepreneurs in the developing world who want to take out small loans to start or expand a business. By spurring economic growth, microfinance groups like this are one of the most effective means of reducing global poverty. Bonus: The vast majority of Kiva's loans are paid back, and your seed money is returned to you in time - which multiplies your donation's effectiveness by enabling you to lend the same money out over and over again.
Added bonus: Kiva's lenders can organize into communities, and right now, far and away the largest lender community on the site is made up of atheists! If you join this group, not only will you be doing good for humanity, you'll be contributing in a very visible way to dispelling the stereotype that atheists lack generosity or charitable impulses.
The American Civil Liberties Union: The fact that Christian conservatives so despise the ACLU ought to be reason number one for politically aware atheists to support it. But if you need more persuasion, consider how many groundbreaking civil-rights victories the ACLU has been involved in - including some that are very valuable to atheists, such as Abington v. Schempp (striking down teacher-led prayers in public school), U.S. v. Seeger (a ruling that the nonreligious can be conscientious objectors), and Epperson v. Arkansas (bans on evolution violate the First Amendment).
Even today, the ACLU is hard at work challenging unconstitutional entanglements of religion and government, as well as battling excessive government secrecy, intrusive police power, laws that restrict free speech, and generally defending the principles that benefit all citizens in a free society. Every American ought to be thankful that they exist, because they're not afraid to defend unpopular causes in the name of protecting the Constitution, and in the long run that benefits all of us.
The Freedom from Religion Foundation: If you want to support an explicitly atheist organization, you couldn't do better than the Freedom from Religion Foundation. An educational and advocacy organization based out of Madison, Wisconsin, the FFRF represents the views of the rapidly-growing segment of the American population that is nonreligious. Like many other groups, they wage legal battles across the country to protect the constitutional principle of separation of church and state; but possibly even more importantly, they work to educate the public about the views of nontheists.
Foundation Beyond Belief is a non-profit charitable and educational foundation created (1) to focus, encourage and demonstrate the generosity and compassion of atheists and humanists, and (2) to provide a comprehensive education and support program for nontheistic parents. The foundation's mission is "To demonstrate humanism at its best by supporting efforts to improve this world and this life; To challenge humanists to embody the highest principles of humanism, including mutual care and responsibility; and To help and encourage humanist parents to raise confident children with open minds and compassionate hearts."
Direct Relief International is a very large organisation that helps disaster areas and is rated as one of the most efficient charities by Charity Navigator, with 99% of funds going to projects with just 1% spent on admin and fundraising. Direct Relief International provides medical assistance to improve the quality of life for people affected by poverty, disaster, and civil unrest at home and throughout the world.
Given that most of our modern problems can be traced to overpopulation, I heartily recommend Population Connection. Since 1968, Population Connection (formerly Zero Population Growth) has been educating young people with its award-winning Population Education program and advocating for progressive action to stabilize world population at a level that can be sustained by Earth's resources.
National Public Radio, the Public Broadcasting System, et al are often the most educational media sources of their kind. They minimize the sensationalism and maximize the depth and breadth of their coverage. PBS always has great documentaries and informational programming. With the constant attack and reduction of funding from the right, they need the money more and more. Donate now and your tax-deductible gift helps keep all your favorite news, entertainment, and music programs on your local public radio station. Your financial support helps ensure that you can listen to Morning Edition, All Things Considered, Fresh Air, Car Talk and Wait Wait ... Don't Tell Me! on your NPR station.
The Atheist's Guide to Christmas is a funny, thoughtful handbook all about enjoying Christmas, from 42 of the world's most entertaining atheists. It features everything from an atheist Christmas miracle to a guide to the best Christmas pop hits, and contributors include Richard Dawkins, Charlie Brooker, Derren Brown, Ben Goldacre, Jenny Colgan, David Baddiel, Simon Singh, AC Grayling, Brian Cox and Richard Herring. The full book advance and all royalties will go to the UK HIV charity Terrence Higgins Trust.
Perhaps the most important site is the meta-site Charity Navigator. Charity Navigator, America's premier independent charity evaluator, works to advance a more efficient and responsive philanthropic marketplace by evaluating the financial health of over 5,400 of America's largest charities.