American Regions Math League (ARML) :
http://www.arml.com/
Princeton University Math Competition (PUMaC) :
https://cgi.math.princeton.edu/mathclub/index.php/Princeton_Math_Competition
Harvard-MIT Math Tournament (HMMT) :
http://web.mit.edu/hmmt/www/
Art of Problem Solving :
http://www.artofproblemsolving.com
Math 55 :
http://www.thecrimson.com/article.aspx?ref=516216
