Google's Ngram viewer allows you to visualize the frequency words have appeared in their scanned titles since 1500.
Google Books is a project to digitize as many books as possible. Though the content in many of the books is copyrighted (particularly in latter titles), the words themselves can be mined/visualized to facilitate research in the Ngrams viewer, basically Google Insights for print. There are a ton of interesting use cases for this, both seeing trends in ideas/concepts and trends in language over time. (And actually, if can be difficult to disaggregate these – as people use more specific terms over time, or language changes over time. And one would like to see a graph of just the amount of words scanned at points in the timeline. Simple queries can lead to some pretty dramatic conclusions…). Be prepared to get sucked in by this tool. Here’s a favorite: