Some links you might find interesting:

• Other math leagues in Massachusetts (all high school)
  • Greater Boston Mathematics League
  • Massachusetts Mathematics League
  • Worcester County Mathematics League
  • Southeastern Massachusetts Mathematics League
  • Southeastern Massachusetts Conference Mathematics League
  • Western Massachusetts Mathematics League
  • Tri-State Math League (MA/ME/NH)
  • Massachusetts Association of Math Leagues

• Math leagues near Massachusetts with websites (again, all high school)
  • Maine Association of Math Leagues
    • Pi-Cone South Math League (ME)
    • Central Maine Math League
  • New York State Math League (NYSML)
  • Nassau County Interscholastic Mathematics League
  • New York City Interscholastic Math League

• Regional/national competitions
  • Massachusetts Mathematics Olympiad
  • American Mathematics Competitions (AMC8, AMC10, AMC12, AIME, USAMO)
  • Mandelbrot Competition
  • New England Math League
  • WPI Mathematics Meet
  • Harvard-MIT Mathematics Tournament
  • Princeton University Mathematics Contest
  • Duke Math Meet
  • American Regions Math League
  • USA Mathematical Talent Search
  • MathCounts (middle school!!)
  • Math Kangaroo (grades 1-12)
  • Webpages containing links to math competitions
    • http://www.mathpropress.com/competitions.html
    • http://www.math.uncc.edu/~hbreiter/

• Nearby enrichment programs
  • Idea Math (Lexington, MA)
  • The Math Club (Lexington, MA)
  • The Math Circle (Cambridge and Boston, MA)
  • Math Round (Swampscott, MA)
  • Art of Problem Solving (online classes)

• Summer math camps
  • MathPath (for middle school students!)
  • Hampshire College Summer Studies in Mathematics
  • Canada/USA Mathcamp
  • Program in Mathematics for Young Scientists
  • Ross Mathematics Program
  • Arteem Institute
  • AwesomeMath
  • Stanford University Mathematics Camp
  • Comprehensive list of math camps