Moems: http://www.moems.org/
 

Mathcounts:  http://mathcounts.org/
American Mathematics Competitions (AMC 8,10,12, AiME, USAMO):  http://www.maa.org/math-competitions/
NC High School Math Contests: http://math.ncssm.edu/smc/state.htm
The Mandelbrot Competition: http://www.mandelbrot.org/

High School Mathematical Contest in Modeling: http://www.comap.com/highschool/contests/himcm/index.html
Duke Math Meet:  http://www.math.duke.edu/dumu/mathmeet/

Harvard-MIT Math Tournament:  http://web.mit.edu/hmmt/www/

American Regions Mathematics League:  http://www.arml.com/index.php

SIAM's Moody's Mega Math Challenge:  http://m3challenge.siam.org/

USA Mathematical Talent Search: http://www.usamts.org

 

 

Virginia Tech Regional Mathematics Contest: http://www.math.vt.edu/people/plinnell/Vtregional/index.html
Putnam Mathematical Competition: http://math.scu.edu/putnam/
Mathematical Contest in Modeling: http://www.comap.com/undergraduate/contests/mcm/