Compulsory, mandatory and obligatory

According to my Oxford Dictionary, these three words can be used as synonyms in many contexts, but there are nuances.

I like the first response in the forum at the following link:

http://answers.yahoo.com/question/index?qid=20080507075259AAR2USG

Here is a link for another forum which can be useful, as well:

http://english.stackexchange.com/questions/2902/mandatory-obligatory-compulsory