AllDictionaries.com
Categorized index of language and subject dictionaries.
http://www.alldictionaries.com

AllWords.com
Features two modern English dictionaries, multilingual search, and translation. Also features a guide to language sites on the web and a language discussion forum.
http://www.allwords.com

Bin's On-Line Dictionary
Search various on-line dictionaries.
http://language.bin.org/ref/dict

Cambridge International Dictionaries
Free online dictionary from Cambridge University Press.
http://dictionary.cambridge.org

Campus Program: Language Dictionaries
A compilation of a large number of dictionary websites grouped by language.
http://campusprogram.com/research/dictionaries/language

The Alternative Dictionaries
Includes a list of slang terms and phrases in various languages.
http://www.notam02.no/~hcholm/altlang