Dr. Thomas Hale is the author of Medications and Mothers' Milk which is a reference book used by doctors and mothers alike. Dr. Hale has a forum where he lists medications and if they are safe to take while breastfeeding. Since this is a forum, other doctors are able to post their professional opinions as well as questions. I have seen many questions and answers regarding medication during pregnancy as well.
I consider this my go-to resource for medications. Of course, I always consult my doctor first but while I wait on him to return my call, I always check out this website.