The Stuttering Foundation.
I had a stutter as a child, I appreciate all that my Speech Therapists did to help me. I had the added stress of moving to different schools frequently as well, which only multiplied the issue.
I wasn't "cured" (though you never totally get rid of a stutter) until I recieved 3 months of intensive therapy at Walter Reed Army Medical Center while in the Marine Corps. It really did change my life.
http://www.stutteringhelp.org/
I also donate time and money to The Human Fund.