Drupal Commerce and Google AdWords Conversion Tracking
Google AdWords provides a tracking code that allows you to keep track of how well your ads are performing - if someone clicks an ad and ends up at the conversion code, the ad was technically a success. Typically you would use this in e-commerce and track order completions. There is a Google AdWords module to allow full Drupal integration...but that seems like a lot of overhead for merely wanting to monitor customer order completion (note: I did not attempt to use the Google AdWords module.)
It seemed much simpler to just create a small module which will add a pane to the checkout form, specifically the "Checkout Complete" page. This pane drops in our Google AdWords conversion code and that's it.