Google's N-Gram Viewer lets users search a 500 billion word database (from 5 million books) for how frequently authors have used a word, from 1800 to the present:
http://ngrams.googlelabs.com/.
Here's a report:
http://www.npr.org/2010/12/16/132106...nge-with-words
And here are searches I ran for "communism", "god", and "internet":
