Very nice site: dictionary WEBSTER online.

This is the site for one of the most famous dictionary of american english DICTIONARY AND THESAUSRUS - MERIAM WEBSTER.
Check the link and find out more about what this webpage provides in terms of information and of course vocabulary.
When you sign up, you are able to receive (if you like it) everyday what they call THE WORD OF THE DAY. It's a very interesting way to get to know more vocabulary in english.
Check it out and lemme know what you think.

http://www.merriam-webster.com/