I recently read that Google plans to have, eventually, all the books ever written scanned and hosted at Google Books. That sounds to me like a lot of work and litigations against the people who own the rights of the books. Anyhow, they have managed to have a huge data base and, taking advantage of it, one can play around with the latest Google Labs creation: Ngram Viewer.

It allows you to compare the number and frequency of occurrences of the words you choose throughout the history, being able to study very interesting correlations and see historical events be reflected in literature.

For example:

Another example I tried is:

A very interesting application by Google. It is also extremelly useful. I know people working towards PhDs in social sciences, anthropology and psychology that use Google Ngram Viewer in their research.