Have you ever wondered how a particular word or words have been used over time?  Google can now give you an idea of word trends throughout the history of a corpus of printed books with their Ngram Viewer.  Enter a word, phrase, or a series of words, and the Ngram Viewer will search through the masses of Google Books to tell you the percentage of use of your query in a sampling of books over a set time span.

For more info on how it works, check out the Ngram about page.