Some of my favorites are as follows (more for umpires):
www.umpire.org (ABUA)
www.amateurumpire.com (Amateur Baseball Umpire Homepage)
www.stevetheump.com (Steve O's Umpire Resources)
www.ecac.com/Officiating/Baseball/ (College Rules Tests, info)
www.stealers.net/quiz/rules_quiz.htm (Baseball Rules Quiz)
http://people.ku.edu/~sdragoje/bball_rules.html (RulesTrivia)
http://www.macroweb.com/ibrules/ (Interactive Baseball Rules)
http://www.hsmaps.com (directions to high schools)
and, of course
www.Redsox.com