NCTM is a website that is very helpful to teachers to meet others and constantly learn new ways of teaching. This is a resource that takes care of the big picture of education.
Council in which math teachers and students planning to teach math can join. This is the national council but there is a local one and that is the North Carolina Council of Math Teachers.
This website is a great resource form information, strategies, best practices, professional development and middle level advocacy. They have a large selection of research summaries and reports to choose from, which are directed at middle grades topics. In addition they have an annual conference, which should be of interest for Middle Level Teachers to attend.