Google Books Ngram Viewer

Another useful tool from Google is Google Books Ngram viewer.

http://googleblog.blogspot.com/2010/12/find-out-whats-in-word-or-five-with.html

This allows one to track the (changing) usage of words.

Some insightful comments are here :

http://datamining.typepad.com/data_mining/2010/12/more-thoughts-on-google-books-ngrams.html