You can find a lot of good monolingual and multilingual dictionaries in the web. Here are some of them which might be useful:

MONOLINGUAL DICTIONARIES:

www.webster.com (or: www.m-w.com)
dictionary.cambridge.org
www.thefreedictionary.com
http://encarta.msn.com/encnet/features/dictionary/dictionaryhome.aspx
www.usingenglish.com (very good for idioms and phrasal verbs)
www.allwords.com
http://definr.com/
http://www.onelook.com

MULTILINGUAL DICTIONARIES:

www.wordreference.com
http://dizionari.corriere.it/dizionario_italiano_inglese.shtml (Italian/English - English/Italian)
http://encarta.msn.com/encnet/features/dictionary/dictionaryhome.aspx (section Translations)

Free Online Language Lessons: Learn Russian, English, Spanish, Italian...