Webster's Revised Unabridged Dictionary
http://humanities.uchicago.edu/orgs/ARTFL/forms_unrest/webster.form.html

Cambridge Dictionaries Online
http://www.clres.com/dict.html#direct

Il dizionario della lingua itlaliana online (De Mauro / Paravia)
http://www.demauroparavia.it/

Dizionari garzanti online
http://www.garzantilinguistica.it/

Roget's 1911 Thesaurus
ftp://clr.nmsu.edu/CLR/lexica/roget-1911

AskOxford
http://www.askoxford.com/dictionaries/

Wiktionary
http://en.wiktionary.org/wiki/Main_Page