There's a huge, green button to the right of the message I'm typing right now.
Other good groups that will make good use of your money are the Freedom From Religion Foundation (http://www.ffrf.org/) and the Society for Creative Anachronism ... err, wait, I mean the Secular Coalition for America (http://www.secular.org/). I get those two confused all the time. Plenty of others, too. I'm sure other people will pop in with a few.
Oh, and Glenn Beck and Bill O'Reilly are assholes. If they're not dishonest, then they're aggressively stupid or ignorant. Pick your poison.
They are both idiots. Why would you even try to like them? lol.
I actively donate to the Secular Coalition for America (secular.org). I have no idea if that's a write-off though since they are a lobby.
I'm positive that Foundation Beyond Belief is a write-off. I also donate to them monthly. They sponsor ten non-religious charities per quarter and you can allocate your donation according to categories that are most important to you. It's a great organization!
Bill O'Rilly thinks because he doesn't understand why we have tides, or sunrises it's evidence of a magical super being. Ta Da!
Glen Beck is an endless source of idiocy to vast for me to even begin to describe.
The fact that either one of them has any credibility highlights the intellectual dishonesty and laziness of a wide swath of the American population.