The list @ohf provided is huge. Some makes sense to me, some doesn't. Outsourcing HR Business Partners most of all, though outsourcing ER also doesn't make a lot of sense. Just my opinion based on working in these areas at other organizations.
HR investigations is not actually an HR function anymore. Employment investigations was split out. It's now under Conduct Management, which rolls up to the COO. CM does the investigation, then forwards the results to ER to work with LOB on the outcome. There are contractors supporting employment investigations, but the general consensus is the company is not getting its money's worth. In house investigators are cleaning up work done by the contracted resources. I'm not saying there the function couldn't be outsourced (if WF could find a quality service provider), but its not an HR function anymore.