Mauro wrote:

you can find some online here: http://pagina1.altervista.org/dictionaries_glossaries_gen.htm (all kinds). You can also download and use WordWeb or Wordnet for free (forever!) at http://wordweb.info/free/ + http://wordnet.princeton.edu/ Good luck, Mauro

Merriam-Webster Online
http://www.m-w.com/
Merriam-Webster provides a free online dictionary, thesaurus, audio
pronunciations, Word of the Day, word games, and other English language
resources.

Dictionary.com
http://dictionary.reference.com/
This is a free online English dictionary, thesaurus and reference
guide, crossword puzzles and other word games, online translator and
Word of the Day.

American Heritage Dictionary
http://www.bartleby.com/61/
Over 90,000 entries feature 10,000 new words and senses, 70,000 audio
word pronunciations, 900 full-page color illustrations, language notes
and word-root appendixes.

Yahoo! Directory Dictionaries > By Language
http://dir.yahoo.com/Reference/Dictionaries/Language/
Yahoo! Search Directory's listing of language dictionaries from
Afrikaans to Yiddish.

RhymeZone
http://www.rhymezone.com/
A language arts reference tool and comprehensive search engine for
words. Includes the functions of a rhyming dictionary, thesaurus, and
spelling checker, as well as an integrated full-text search engine for
all of Shakespeare's works and thousands of quotations and poems."

On-Line Medical Dictionary
http://cancerweb.ncl.ac.uk/omd/

MedTerms Medical Dictionary
http://www.medterms.com/script/main/hp.asp

ArtLex Art Dictionary
http://www.artlex.com/
You'll find definitions for more than 3,500 terms used in discussing
visual culture, along with thousands of supporting images,
pronunciation notes, great quotations and cross-references.

SYMBOLS.com
http://www.symbols.com/

Webopedia
http://www.pcwebopedia.com/
An online computer dictionary and internet search engine for computer
and Internet technology definitions.

Dictionary of Algorithms and Data Structures
http://www.nist.gov/dads/