2 American Heritage
[FREE] http://www.ahdictionary.com/
=The American Heritage® Dictionary of the English Language, 5th Edition
+The American Heritage Dictionary of Idioms, 2nd
[FREE] http://www.thefreedictionary.com/
=The American Heritage® Dictionary of the English Language, 4th Edition
3 Longman
[FREE 30-day trials] http://global.longmandictionaries.com/
=Longman dictionary of contemporary English 6th Edition
+Longman Collocations Dictionary and Thesaurus
[FREE 30-day trials] http://longmandictionariesusa.com/
=Longman Dictionary of American English - 5th Edition
+Longman Advanced American Dictionary 3rd Edition
+Longman Thesaurus of American English
+Longman Study Dictionary of American English 2nd Edition
5 Collins
[FREE] http://www.collinsdictionary.com/
=Collins COBUILD Advanced Learners Dictionary
+Collins COBUILD Advanced Dictionary of American English
+Collins English Dictionary
+Collins American English Dictionary
+The Collins English Thesaurus
+The Collins American Thesaurus
[FREE] http://dictionary.reference.com/
[FREE] http://www.thefreedictionary.com/
=Collins English Dictionary - Complete & Unabridged 10th Edition