There are also a number of companies (some in the S&P 500, more of them not), that get all or most of their revenue by providing services to the "obviously" oil & gas companies. You've got large steel and railroad companies, then you've got small contractors providing welders/machinists/millwrights/etc.

Indeed. I have personally worked with two such companies that sold equipment and services to oil and mining companies. Both those sectors have been hit hard and their suppliers (esp. ones who had not diversified well) were hit in turn.