These are the animal organizations that get my contributions:
my local SPCA (check the phone book or web to find where they are local to you)
The Morris Animal Shelter - a local, nationally reknown shelter
A local no-kill cat shelter (there are probably no-kill shelters in your area as well)
Humane Society of the United States - I've given to them in the past, but I did not donate last year, and am keeping a wary eye on a few of their current projects. I haven't written them off yet, but neither have I written them a check yet.