General Information

Mathematics

Mathematical Association of America (MAA): For mathematics students, the MAA provides information on a number of topics.
American Mathematical Society (AMS): AMS maintains an archive of information for math majors.
Society for Industrial and Applied Mathematics (SIAM): SIAM supports a number of activities for undergraduates, including conferences and student chapters.
Association for Women in Mathematics (AWM): AWM has a main focus encouraging women to study and have active careers in mathematics.
Young Mathematician's Network: The Young Mathematician's Network website has a page on "undergraduate life", with links to articles and to most of the sites maintained by the professional societies. Also, students can post questions/comments about various topics.

Association for Computing Machinery (ACM): For computer science students, each of the main professional societies encourages student participation and provides a variety of educational and career services.
Computing Research Association (CRA)
Institute of Electrical and Electronics Engineers (IEEE)