Quick Links
General Links:
Mu Alpha Theta: http://www.mualphatheta.org/Default.aspx
In-School Tournament Links:
USAMTS: http://www.usamts.org/
Math League: http://www.mathleague.com/
Mandelbrot: http://www.mandelbrot.org/
Log1: http://log1.wamath.net/
Interstellar: http://in-ter-stel-lar.com/learn
Rocket City: http://rocketcitymath.org/
Math Kangaroo: http://www.mathkangaroo.org/2010page/kangur/main.htm
University Tournament Links:
Harvard/MIT: http://web.mit.edu/hmmt/www/
Rice University: http://www.ruf.rice.edu/~eulers/RMT.html
UIL Links (including practice tests):
Number Sense: http://www.uiltexas.org/academics/number-sense
Calculator Applications: http://www.uiltexas.org/academics/calculator-applications
General Math: http://www.uiltexas.org/academics/mathematics
TMSCA Website: http://www.tmscaonline.net/
Even more practice tests: http://texasmath.org/_downloads.html
Other:
Art of Problem Solving: http://www.artofproblemsolving.com/
Brilliant: http://brilliant.org/
MathCounts: http://mathcounts.org/
Ole Miss Math Challenge: http://mathcontest.olemiss.edu/