Science and Industry Advance with Mathematics (SIAM)- This is a great source of information. It has career profiles, companies that hire mathematicians, and general good information.

The Young Mathematicians' Network - A website for young mathematicians. (i.e. us!)

Paul's Online Notes - Covers Alegbra, Calculus I-III, Differential Equations and Linear Alegbra.

Khan Academy - Free Video Lectures/Notes that covers basic arithmetic, Alegbra I-II, Geometry, Precalculus, Trigonometry, Statistics, Probablity, Calculus and Linear Alegbra.

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Halliburton

Inroads - Helps students find an internship. Application and acceptance required.

American Statistical Association

Math's Careers UK - UK website but still generally good info.

Mathematical Association of America

American Mathematical Society

NSA - "The largest employer of mathematicians"