The most obvious dictionary on the web is dictionary.com - this site publishes dictionary reference information from a range of dictionaries including common word dictionaries as well as specialist reference works to elaborate on certain words. Also available on dictionary.com is an online thesaurus which can be found at thesaurus.com Dictionary.com also includes a handy phrase translation tool to and from English for a range of modern languages.
For school pupils in UK, your favorite one of your university dictionaries are available online too at: Cambridge Dictionaries Online
The most definitive and authoritative record of the english language is the Oxford English Dictionary. You will need to register a subscription to use their web site.
For new words try bartleby.com, Over 10,000 new words are included from several reference works, where you can buy the printed version online at this site. Also for new words, have a look at Microsoft Encarta - this is a huge interactive learning site based in the Encarta Multimedia Encyclopedia, the dictionary includes many modern words not found elsewhere.
After trying all the serious dictionaries above, for an alternative way of looking up words, why not try The Visual Thesaurus. You can search on a word on this site and it will generate a diagram with your word in the middle with similarly words on a diagram flying round it. The distance of the other words is an indication of closeness of meaning. Gimmick? Maybe, Perhaps, Possibly, Perchance ... TRY IT.
Most of the dictionaries mentioned above are mainly english language base. For a comprehensive multi-language resource, go to Your Dictionary.com. This site has references covering most modern languages as well as a large resource and links to other sites for researching lesser known languages.
Finally, if you like the one-stop-shop type of web site, use OneLook Dictionary Search. When performing a search on this site, results are drawn from several online dictionaries.
On a lighter note, but still useful for serious language research is London Slang which has a comprehensive list of slang words and phrases originating from London. For slang and colloquialisms from the whole of the UK there is the Slang Dictionary - a massive resources on UK slang. The Rap Dictionary is a large resource of words and terms used in Rap and Hip Hop music, the sites includes information on rap artists as well as rap terminology.
To search for biographies on famous people by name, The Biographical Dictionary has a database of over 28,000 people from ancient times to the present day. The comprehensive search facility allows searching by birth years, death years, positions held, professions, literary and artistic works, achievements, and other keywords.
Grove Art Online is a comprehensive resource on the visual arts. There are over 45,000 articles covering fine arts, decorative arts, and architecture dating from prehistory to the present day.
If you want to become an expert on Jewellery, help is at hand at All About Jewels - an illustrated dictionary about jewels and jewellery.
For any of you that are confused by most modern computing terminology (i.e. do you know your RAM from your ROM, why do techies snigger at 0xDEADBEEF), a useful lookup site is FOLDOC which has a very comprehensive list of computing terms. To find out the meaning of terms related to the World Wide Web try Webopedia.