Merriam-Webster Online   http://m-w.com/home.htm The Merriam-Webster Dictionary and Thesaurus, plus other features.

Onelook http://www.onelook.com  An excellent site containing links to many dictionaries of all kinds, both general and specialized.

Smiley and Acronym Dictionary by Gary Webb http://www.itsg.u-net.com/smileys.htm A dictionary of emoticons and acronyms for use in e-mail messages.

Travlang's Translating Dictionaries http://dictionaries.travlang.com/ Translating dictionary.  You can translate from Latin to English, for example. This should be a good site for translating foreign terms you encounter in your philosophy readings (when snobbish editors don't bother to provide translations!

Web of Online Dictionaries http://www.yourdictionary.com/ Site containing mostly language dictionaries for translating words (everything from Afrikaans to Yiddish!).  Also contains thesauri and specialized English dictionaries.  Extremely useful.