Replaces all instances of BTC with their equivalent in mBTC, because BTC costs so much and tiny numbers representing huge amounts of USD is not the best way to represent Bitcoin.
For example, if you look up ".5 BTC to USD", all instances of ".5 BTC" will be changed to "500 mBTC".
Here is a direct link to download the extension. To install, go to the Chrome extension page (chrome://extensions/) and drag the downloaded file onto the page.