WeUseMath.org is an invaluable guide to math careers of all types.

The Early Career Profile Network from the AMS profiles the first careers of recent undergraduates.

BeAnActuary.org from the SOA

Career Profiles of Math Majors from the MAA

Career Information from the AMS

Careers in Statistics from the ASA

Thinking of a Career in Applied Mathematics? from the SIAM.

National Security Agency - a big employer of the mathematically minded

Why Major in Mathematics? from Duke University

Women at NASA

Careers in Operations Research or Management Sciences

Undergraduate math conferences and journals:

Hudson River Undergraduate Mathematics Conference

Rose-Hulman Undergraduate Math Journal

Council on Undergraduate Research

Summer math programs and internships:

Research Experiences for Undergraduates (REU's) from the NSF

Internship Opportunities for Undergraduates maintained by the AMS

Park City Math Institute Undergraduate Summer Math Program

Carleton College Summer Mathematics Program for Women

George Washington University Summer Program for Women in Mathematics

Semester long math programs:

Mathematics Advanced Study Semester (MASS) at Penn State University

Center for Women in Mathematics at Smith College

Math in Moscow

Budapest Semester in Mathematics

Seeking help and advice:

Siena Career Center

Finding an Actuary Job from the SOA

Job Postings:

MAA Math Classifieds. A job postings website run by the MAA.

CAS Career Center from the Casualty Actuarial Society

SOA Career Center from the Society of Actuaries

ASA JobWEB for jobs in statistics

SIAM Job Board from the Society for Industrial and Applied Mathematics

National Security Agency Job Board - a big employer of the mathematically minded

Giving back through teaching:

MathforAmerica.org

New York City Teaching Fellows

Teach for America

Siena Career Center can help navigate the grad school application process.

Graduate School FAQ from Purdue University

Preparing for Graduate School in Mathematics from Saint Louis University

Choosing the Right Grad School from Harvard University

Applying to Graduate School from the AMS

Graduate Student Blog from the AMS

Career Advice from Terry Tao

Advice for Graduate Student in Statistics from J. Michael Steele

American Mathematical Society (AMS)

American Statistical Association (ASA)

Association for Women in Mathematics (AWM)

Association of Mathematics Teachers of New York State (AMTNYS)

Mathematical Society of America (MAA)

National Association of Mathematicians

National Council of Teachers of Mathematics (NCTM)

Institute for Operations Research and the Management Sciences

Society for Industrial and Applied Mathematics (SIAM)

Society of Actuaries (SOA)

The Society for Mathematical Biology