I found this article recently in the Washington Post ?How to Be Routinely Charitable. http://www.washingtonpost.com/wp-dyn/content/article/2007/10/18/AR2007101802046.html
On the surface, it sounds like some good advice, but as always, buyer beware.
Working Assets (http://www.workingassets.com) does not allow you to determine what they support. The organizations they are supporting are listed at http://www.workingassets.com/Recipients.aspx and while there are some you and I may support, there are many you may not want to support with your dollars.
The GivingExpress Program (http://www.americanexpress.com/give) allows American Express cardholders to make donations to a charity of their choice through automatic donations, redeeming Rewards, etc. And just in case you were worried too much of your donation would go to the cause you want, 2.25% is deducted for processing costs, although the full amount is tax deductible.
That's the bad, here's the better...
For online shopping, iGive (http://www.igive.com) will donate a portion of the sales to the charity of your choice. It seems pretty flexible, lots of choices.
And try out GoodSearch (http://www.goodsearch.com), which gives a portion of proceeds to your charity and has discounts at many retailers.
And perhaps the best and least used:
Frequent flier miles can be donated to Make a Wish or the Red Cross.