http://toughmudder.com (upcoming dates include: Boston, Oregon, Michigan, UK, Germany, Australia):
http://www.runforyourlives.com The Zombie Race (upcoming dates in Dallas, New Jersey, Indiana, Colorado):
http://www.ironwarriordash.com :
http://www.spartanrace.com :
http://muddybuddy.competitor.com/ (this one seems safe and easy enough for kids too):
