Quite an interesting dictionary to check out new expressions, idioms and the like. An interesting feature of the dictionary is that if you look up a common verb such as know and click on idioms a comprehensive interactive list of idioms and collocations is displayed.


http://idioms.thefreedictionary.com/know
http://idioms.thefreedictionary.com/on+its+last+legs
http://idioms.thefreedictionary.com/lost+for+words

Great tool to create your own mind maps and practice definitions using two related words.
http://ninjawords.com/

Some excellent online dictionaries:

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

For slang and informal English

http://www.urbandictionary.com/

Visual dictionary

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

Very good online dictionary for those interested in informal English and slang. This resource has been created by internet users rather than by linguists. Unlike traditional dictionaries which tend to feature outdated slang, this online dictionary is filled to the brim with terms people use and abuse on a daily basis.

http://www.urbandictionary.com/define.php?term=posse

Here's a fine MONOLINGUAL DICTIONARY.

http://idioms.thefreedictionary.com/True+Blue