American Mathematical Society
The American Mathematical Society is the major North American mathematical society, with 30,000 members, whose mission is to "further the interests of mathematical research and scholarship", see http://www.ams.org/ams/ams-report.pdf
European Mathematical Information Service
The European Mathematical Information Service supports mathematical research in Europe including participating in making globally accessible many important scholarly resources.
Mathematical Association of America
The Mathematical Association of America focuses on improving undergraduate mathematics education.
Society for Industrial and Applied Mathematics
The Society for Industrial and Applied Mathematics promotes the development of applied mathematics and computational mathematics to solve problems in engineering and science.
AMS Mathematics on the Web: Societies, Associations, and Organizations
List with links to mathematics societies and major science societies around the world.
Math Archives: Mathematics Institutes, Centers, and Laboratories
List of mathematics institutes around the world.