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.
Silicon Valley Dictionary - Urban dictionary meets Silicon Valley
SlangSite.com - SlangSite.
Reverse Dictionary - Describe a concept and get back a list of words
InternetSlang.com - InternetSlang.
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.