Reverse Dictionary - Describe a concept and get back a list of words
Urban Dictionary - Urban Dictionary is a Web-based dictionary of slang words and phrases.
Wordd - Quickly look up vocabulary, abbreviations, acronyms, slang
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.
Silicon Valley Dictionary - Urban dictionary meets Silicon Valley
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.