Unable to find the chromedriver executable. Please download the server from http://code.google.com/p/chromedriver/downloads/list and place it somewhere on your PATH. More info at http://code.google.com/p/selenium/wiki/ChromeDriver. (Selenium::WebDriver::Error::WebDriverError)
On Ubuntu 13 with Watir and Ruby.
For other people on MacOS:
brew cask install chromedriver
And you'll be good to go.