Networking with one's fellow scientists and engineers is extremely important for personal and professional development. And even in the world of Web social networks, professional societies are the most important means of doing so. Professional societies sponsor the conferences you will wish to attend or at which you will present papers. They publish journals which you will read, submit papers to, or serve as a reviewer or editor. They set professional and educational standards. They provide job and career services for their members.
To find societies not listed below, try the Scholarly Societies Project (http://www.references.net/societies/index.html/) While the site has not been recently updated, it is still a good starting point for tracking down scholarly associations and societies.
Note: The lists below include only societies which currently have a website; generally, if the national language uses the Latin alphabet, the name is given in the local language; if it uses a non-Latin alphabet, the name is given in English translation.
This section is for societies and associations headquartered in the United States. In most cases, their membership is international is scope.