Wordcount.org

Wordcount.org (http://wordcount.org/main.php) is a cool and fun way to explore the English language. It's an interactive representation of more than 86,000 English-language words and how often they occur in writing and speech. The underlying data comes from the British National Consortium, a consortium of universities and publishers led by Oxford University Press.
The website has also added Querycount (http://wordcount.org/querycount.php), which represents the number of times certain words are searched in the Wordcount graph.