I've been a donor and volunteer at Alexandria Tutoring for the last five years. The help teach K, First and Second graders in the Alexandria public school system to read. One thing I like as a donor is that they keep you up to date each year on how well the kids are doing. (Generally 85%+ make it to grade level.) They are also good about letting you know how much of your donations go to the program (~90%).
I can't recommend them more highly.