I give what I can to Saint Jude's Children's Hospital, American Association for Cancer Research (through Calvary Hospital/Hospice) and Graymoor (a Catholic shelter for homeless men in Garrison, NY) on a yearly basis. These are all reputable charities. But I tend to agree with Geoffy about doing research.
A few years ago I bought some candy (I know, no big deal) from these kids outside the A&P. They told me it was for a basketball team (uniforms and traveling expenses). About a month later, they were exposed as a total fraud. One of the kids' parents masterminded the whole thing. They brought in over 40 thousand dollars!
I felt like a total idiot, even though I spent less than 60 dollars. But it taught me a valuable lesson. Not even "innocent" looking children can be trusted when they claim to be doing charity.