Google Books has a vastly cool new feature, the Books Ngram Viewer, which lets you track the relative frequency words in various Google Books databases. So if you want to track the usage in American English of camaraderie and comradery (an inquiry that arose in this comment thread), just run this query and you’ll see. Or if you want to track defense and defence, again in American English, run this query. And of course you can track individual words as well.
Thanks to Chris Hundt and Josh Blackman for the pointer.