Some Useful Sites For Writers - Ngram

Google Ngram Viewer

Unbelievably useful tool for checking whether certain words are anachronistic when writing historical sequences. Seriously between this site and Wikipedia no script writer has any excuse for peppering their dialogue with words and concepts that belong to the modern age. I'm looking at you Sleepy Hollow!
Essentially it's a tool for discovering word frequency amongst a corpus of 5 million books, searches can be differentiated by language and time period. If you need to know whether your 18th Century British emigre uses the word phrase 'learning curve' in casual conversation than this is the tool for you.
Google have wasted no money on making this site user friendly but once you've mastered the joys of late 80s Boolean search commands you'll never look back. 
An important caveat is that words can change their meaning over time - thus in the early 18th Century the word 'projector' referred to people that promoted financial or industrial schemes. It's technological meaning doesn't arrive until later.