Type in a word or phrase in one of seven languages (English, French, German, Spanish, Hebrew, Russian, Chinese) and see how its usage frequency has been changing throughout the past few centuries. Addictive. 
Note: the dataset's quality is highest for English between 1800 and 2000. Some more on interpretation.

The Google Books Ngram Viewer plots are pretty, but you need the data? Use this application.

We introduce culturomics: the application of high-throughput data collection and analysis to the study of human culture.
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On December 16th, 2010, a team spanning the Cultural Observatory, Harvard, Encyclopaedia Britannica, the American Heritage Dictionary, and Google published a paper describing the Culturomics approach online in the journal Science, and at the same time launched the world's first real-time culturomic browser on Google Labs.