Urban Dictionary - Urban Dictionary is a Web-based dictionary of slang words and phrases.
The Online Slang Dictionary - Learn thousands of real slang words or add your own.
Reverse Dictionary - Describe a concept and get back a list of words
Wedefine - Wedefine is an open source dictionary site that you can self host
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.
Hinglish Dictionary - HinglishDictionary.com is an archive of user submitted Hinglish words. Users can register and create accounts and add new words, that must have a touch of Hinglish language in it.