http://www.ams.org/mathweb/mi-sao.html

What I would recommend is not necessarily looking at going for the "big ones", which happen to be AMS and MAA, but looking for ones that match your interests, or searching within the big ones for a Special Interest Group that matches your interest.  

I've probably found more side organizations that I'm not a member of, but were very interesting.  Like the Association for Symbolic Logic or the Kurt Godel Society.

But you might find your niche somewhere else ;).