Art of Problem Solving (AoPS) - forums, problems, lectures, information and courses on a wide range of topics and levels.
Math StackExchange - Q & A forum for a wide range of mathematics
Wolfram MathWorld - math encyclopedia - for advanced students
Khan Academy - thousands of short instructional videos
The Math Forum - wide range of math resources
Curriculum Inspirations - multimedia experience from MAA for middle-school and high-school communities
National Math Club - provides resources and materials to run a math club
Planet Math - online encyclopedia
Plus Magazine published by Cambridge University
Association for Women in Mathematics
ArXiv - open access to over 800,000 e-prints in math and math-related fields.
American Mathematics Competitions - AMC 8, AMC 10, AMC 12, AIME, USAJMO, USAMO, and IMO (International Mathematical Olympiad). This set of competitions is for students through 12th grade.
American Regions Mathematics League (ARML)
USA Mathematical Talent Search (USAMTS) – proof based, online, untimed.
Math Prize for Girls - for high-school girls
Harvard MIT Math Tournament (HMMT and HMNT)
Princeton Math Competition (PUMaC)
Math League - runs various state and regional math competitions at elementary, middle, and high school levels.
Mathcounts - for middle school students
Math Video Challenge - team competition for middle school students
Bay Area Math Olympiad (BAMO)
Mu Alpha Theta - runs regional, state, and national math contests.
Siemens STS Competition – research competition
Intel Science Talent Search – research competition
Purple Comet! Math Meet - online, international, team competition
European Girls Mathematical Olympiad (EGMO) - international competitiona
China Girls Mathematical Olympiad (CGMO) - international competition
International Tournament of Young Mathematicians - international open problem team competition
AoCMM - Online team contest of mathematical modeling
Ross Mathematics Program
Hampshire College Summer Studies in Mathematics (HCSSiM)
Research Science Institute (RSI)
Minority Introduction to Engineering and Science (MITES)
Stanford University Mathematics Camp (SUMaC)
Honors Summer Math Camp
All Girls, All Math
MathZoom Summer Camp
Williams College Math Camp (WCMC)
Math Path - for middle school students.
Camp Euclid - online research program for middle and high school students
Simons Summer Research Program - STEM research program for rising seniors
Modern Mathematics International Summer School for Students (MOMISSS)
International Summer School of Mathematics (EIVM) - run by the University of Seville, the Euler Foundation, and Saint Petersburg State University
The American Mathematical Society's listing of high school summer programs is available here.
PRIMES - year-long mentored research program.
MIT Open Courseware - MIT course materials online.
Udacity - online courses in math and science.
Coursera - online courses.
Math Circles - click here to look for a math circle near you.
Animath - online French mathematics resources and contests.
NRICH - online mathematical resources and forums in the UK.
KöMaL - Exercises and problems in Mathematics, Informatics and Physics for advanced high school students.
Estalmat - program for mathematically gifted students in Spain.
Berlin Network of Schools Specializing in Mathematics and Science
Germany Mathematical Society (DMV)
Math en Jeans - team research program in France.