Sometimes you come across apps and tools on the web that just serve the purpose they were designed to do and Wordnik is definitely a brilliant example of this. Wordnik is quite simply the best online dictionary site and should be in the favourites bar of every teacher and student.
Wordnik is a true 2010 web dictionary. For example if you search the word cat it will give you all of the stock standard dictionary definitions and meanings on the word but you will also find examples of using the word correctly in a sentence from various articles across the web.
You will also find an audio file for correct pronunciation and flickr photos relating to the word. To further this you will also find data relating to the use of the word and references to it in Twitter.
All in all Wordnik has taken the role of the Oxford style of dictionary which has been pretty stagnant for nearly 200 years and introduced a range of web 2.0 technologies to enhance it's use as a learning and reference tool. Check out Wordnik here.