Any one that saves an animal?
Which rules out PETA and Humane Society of the United States. Two outfits that don't help very much. Stay away from PETA, they don't believe in keeping pets and kill every one they can get their hands on (even if they have to steal one from your porch), and HSUS doesn't spend much of its money on shelters, it's a political action group that lobbies for a living.
I like our AZ Humane Society quite a bit, they have a no-kill shelter.