University of Texas links

The University of Texas at Austin the home page of the University
Department of Mathematics includes links to math people and courses at UT as well as to many external resources
The Saturday Morning Math Group aka SMMG, a group that meets about once a month to expose middle- and high-school students to interesting extracurricular mathematics
Math Contests at UT The Albert A. Bennett Calculus Prize Exam and The William Lowell Putnam Mathematical Competition; includes information on practice sessions for those interested in taking the Putnam
Math Club Forum a place for math club members and friends to discuss various math topics, offer tutoring services, request help with difficult classes, post interesting math-related news articles, etc.

National Organizations

AMS The American Mathematical Society
MAA The Mathematical Association of America
SIAM Society for Industrial and Applied Mathematics
AWM The Association for Women in Mathematics
List of REUs available through NSF website

Fun, Puzzles, and Games

Project Euler a collection of math-related programming exercises, ranging in difficulty from easier to harder
Matchstick puzzles at zefrank.com A small collection of matchstick puzzles
math riddles a huge assortment of hardcore tech-interview style riddles and mathematical puzzles, and also daily high-quality forum discussions.