The Ngrams web viewer has been updated. This amazing tool allows you to search for word frequencies in Google Books' huge archive of digitised text. You can choose British or American English, choose what period of English you're interested in, and search for several terms which will appear as different coloured frequency lines on the same graph.
New to the search engine is the ability to search by part of speech (word class), as well as other special functions as explained in the link below. They've updated the data to 2012 as well — but you can still find the old 2009 data. Choose your preferred data set from the drop down menu.
There's lots more advice and tips in the blog post: