• http://www.merriam-webster.com/

The reference dictionary of US English

• http://www.gutenberg.org/                                                    

Thousands of free classical books to download

• http://www.urbandictionary.com/

A dictionary of Slang (US)

• http://www.peevish.co.uk/slang/index.htm

A other dictionary of Slang (UK)

• http://www.askoxford.com/

Excellent site for English student and all people who are passionate about language