Urban Dictionary - Urban Dictionary is a Web-based dictionary of slang words and phrases.
Dictionary.com - Dictionary.com is the world’s leading digital dictionary. We provide millions of English definitions, spellings, audio pronunciations, example sentences, and word origins.
Reverse Dictionary - Describe a concept and get back a list of words
ManyThings.org - ManyThings is a multiplatform and user friendly application for students of ESL or EFL to learn the meanings and definitions of different terms and phrases.
Dictionary Anywhere - Dictionary Anywhere is an open-source and easy-to-use browser extension for Mozilla Firefox and Google Chrome that allows you to check definitions easily while browsing the web.
The Online Slang Dictionary - Learn thousands of real slang words or add your own.