Interested in tracing the historical usage of a given word or phrase? Maybe you'd like to compare the usage of two synonyms over time? How about visualizing historical technological developments? Google's Ngram Viewer lets you do exactly these kinds of things by searching for your terms in within the pages of "10% of all books ever published" (i.e. the books Google has scanned). You can even specify the date range of your search.
I don't know that this new application from Google will ever have any particular relevance to libraries but it is fun to play with. Here's a graph showing the use of the phrase "public library" over the past several centuries. How about that trend starting around 1980?