At your 15 month appointment, ask your pediatrician to refer you to Early Intervention. Our son is currently seeing a speech therapist because he, too, wasn't saying really anything at 15 months. They will assign you a case coordinator and do a home visit where they test to see in which areas your child is demonstrating delays. Don't worry about the 'test' though; they basically bring a bunch of toys for your kid play with (stacking rings, manipulatives, a baby doll, among other things), and make observations through their interactions with your child. My son had so much fun during his evaluation!
After the evaluation, they will determine whether your child is demonstrating a delay and make arrangements for therapy if necessary. Although the therapy sessions are not free, they are based upon your income and a 'sliding scale' is used. You also have the option to have All Kids cover the costs, but since our family is not eligible I don't know much about that.
Our son qualified for an hour weekly with a speech therapist and it has been wonderful. I'm not the type of mom who wants her son to be reading War and Peace and giving dissertations at the tender age of 18 months, but I did want him to be speaking at a 'developmentally appropriate' rate for his age. The therapy sessions are great because not only does our son learn more, I also get to watch the therapist and learn how I can help my son.
I know there are some moms out there who just say "outgrow it; they'll talk eventually". To me, I felt there was always the possibility that this developmental delay could snowball into other issues and make things worse as he gets older (I'm a teacher and have seen what happens to kids whose parents ignore their early signs of difficulty, then hit the panic button in high school when it is much more difficult to get back on track).