The Google Ngram Viewer is a phrase-usage graphing tool which charts the yearly count of selected n-grams (letter combinations), words, or phrases, as found in over 5.2 million books digitized by Google Inc (up to 2008). The words or phrases (or ngrams) are matched by case-sensitive spelling, comparing exact uppercase letters, and plotted on the graph if found in 40 or more books. The Ngram tool was released in mid-December 2010.
The word-search database was created by Google Labs, based originally on 5.2 million books, published between 1500 and 2008, containing 500 billion words in American English, British English, French, German, Spanish, Russian, or Chinese. Italian words are counted by their use in other languages. A user of the Ngram tool has the option to select among the source languages for the word-search operations.
Search engine: http://books.google.com/ngrams
TED Talk about Google Ngram Viewer: http://www.ted.com/talks/what_we_learned_from_5_million_books.html