On-Line Encyclopedia of Integer Sequences
You give it an integer sequence, it will search where in math that sequence appears.
Math Resources
arXiv
Open access to lots of math e-prints.
Wolfram Mathworld
Pretty good resource for looking up mathematical definitions, etc.
Project Euler
Collection of mathematical and programming problems.
SageMath
Free open-source mathematics software system.
MAA
The homepage for the Mathematical Association of America. "all math majors get a membership"-anon.
AMS
The homepage for the American Mathematical Society.
AoPS
The Art of Problem Solving is a good resource for problem solving.