English and American
Onelook - gives quick, very simple definitions.
The Oxford English Dictionary - still the best, but not free - you have to subscribe to this service.
Even More Foreign
YourDictionary - offers cross-language dictionaries for a large range of languages - scroll down the front page for the list.
Phrase, Fable and Idiom
Prodigy - various sources: pick up Brewers Phrase and Fable, Britspeak
Netlingo - computer techno-speak
Whatis - claims to define every internet term and file in the world, and you can even go on their email list to keep up to date.
The Times Online Style Guide - aimed at journalists, with sections on special areas such as the Church, politics, the courts, sport and the armed forces.