This video blog highlights difficult decisions made in the MLS with a brief review. A good resource for new and experienced referees, particularly in the areas of judging Serious Foul Play, Feigning/Diving, and Offside.
FIFA Refereeing Assistance Program (FIFA RAP)
Check out this great summary of considerations that a referee must make on the field to make a judgement for several footballing concepts.
US Soccer Box & FIFA DVDs
A Referee (Tricky) Online Quizzes
This is a good place to train your mind in preparation for LotG quizzes or just general LotG knowledge. http://areferee.com