Merriam-Webster Online
You can install the Merriam-Webster Toolbar into your browser free of
charge. This gives you instant browser access to a dictionary and thesaurus (only works with Internet Explorer version 5.0 or higher).

The Word Spy
This Web site and its associated mailing list are devoted to recently coined words, existing words that have enjoyed a recent renaissance, and older words that are being used in new ways.
Each weekday, the Word Spy presents a new word, its definition, and a citation (usually from a major newspaper or magazine) that shows how people are using the word. You also get extra goodies such as background on the word's formation, a list of related words from the Word Spy database, quotations on words and language, and more.
So, if you want to know the meaning of 'acoustic terrorism', 'time porn' and 'resistentialism', this is the place to go...