On-line dictionaries (Kasia Marcinkiewicz, IOU)

Cambridge Dictionary Online

Longman Dictionary of Contemporary English

Oxford Advanced Learners Dictionary

www.visualthesaurus.com - like a mind map

http://www.m-w.com/

TheFreeDictionary

WordReference.com

Webster's Online Dictionary

The American <-> British Dictionary - a quite good reference to differences between American and British language

www.urbandictionary.com  - slang dictionary where u can find many interesting and funny words and phrases with clear explanations and examples.

Let's play - http://www.miniclip.com/games/hangaroo/en/