PhantomJS is an awesome tool for doing automated, full-stack acceptance tests in a headless browser. It's a great alternative to capybara-webkit because it doesn't depend on QT or xvfb, and it's way better than Selenium in most cases because it's faster (you don't need to open Firefox to run your tests).
Unfortunately, installing PhantomJS via Homebrew can wreak havoc on your Rubies if you're using rbenv. These are the steps that worked for me in fixing my Ruby installations.
# Update Homebrew
brew update
# Install PhantomJS