AMC8: https://www.maa.org/math-competitions/amc-8
AMC10/12: https://www.maa.org/math-competitions/amc-1012
MathCounts: http://www.mathcounts.org/
MathCounts Handbook: http://www.mathcounts.org/resources/school-handbook
MathCounts Mini Videos: http://www.mathcounts.org/resources/video-library/mathcounts-minis
San Diego Math Field Day: http://www.sdmathfieldday.com/
MOEMS: http://www.moems.org/
Math Kangaroo: https://www.mathkangaroo.org/
Art of Problem Solving (AoPS): http://artofproblemsolving.com/resources
AoPS Video Lectures: http://artofproblemsolving.com/videos
AoPS AMC8 Problems & Solutions: https://artofproblemsolving.com/wiki/index.php?title=AMC_8_Problems_and_Solutions
AoPS AMC10 Problems & Solutions: http://artofproblemsolving.com/wiki/index.php?title=AMC_10_Problems_and_Solutions
AoPS Math Counts Trainer: https://artofproblemsolving.com/mathcounts_trainer